Probably the best way to figure out your decks' deficiencies is to show it to others for critique. Like posting them in this forum's Casual section. Learn about the fundmentals of deck building, that players have figured out over the course of two and a half decades that the game existed, and implement them. I'll try to summarize them:
1) Have a plan, and have only one plan. Combining strategies or putting in a secondary strategy "just in case" weakens your main strategy as fewer cards are dedicated to it. And the secondary will never be as good as it could be if it were the sole approach, and for the same reason: only part of the deck is dedicated to it.
2) Play full sets of four of your key cards to increase consistency of drawing them. A good deck wants play the same each time as much as possible.
3) Stay at or at least close to the minimum deck size to increase consistency of drawing your key cards.
4) Good decks play good cards. All cards are not created equal, you want to get the most out of every single card slot in the deck. Playing subpar and/or fringe cards just won't cut it in the long run.
5) Play enough lands. Each deck has its own land needs, but most require about 40% of th deck to be lands. That means about 24 lands in 60 cards. Aggro decks can cope with a bit less (though most of those still shouldn't go below 20), Control decks may need up to 28 or even more. (Yes, almost half the deck.) Being too greedy with your lands will hurt the deck much more than the additional spells will help it.
6) Interact with your opponent. You are not playing solitaire (though some combo decks can feel like that), your opponent is trying to win as much as you do. You need ways to hinder his plans, and expect him to hinder you. At the very least, you should have some ways to kill or otherwise neutralize creatures, but discard, counterspells, library manipulation, etc. all fall under this point.
7) Pure life gain is BAD. If you are running cards that simply give you some life and offer nothing more, you are wasting card slots. Gaining life is like patching up wounds while your opponent keeps stabbing you. It's much more efficient to kill the thing that's killing you, or to stay within the metaphor, knock the knife out of his hands. Life gain should alsways come as an additional effect or option, not be the sole purpose of a card. Also, life is a resource, you don't have to protect every single point, only the last one matters. So don't throw valuable creatures in front of attackers if you can take the hit instead.
8) Pay attention to your deck's mana curve. Look up the phrase if you want more details. But in short, the mana curve refers to how many cards you have for each mana cost. The bulk of those should usually be at 3-5 mana with some cards for less mana for the earlier game, and higher cost cards being rarer, since you can't play them early and you don't want them uselessly cluttering your hand. You don't have to play stuff during the early turns, but you need to be able to.
9) Try to get card advantage. Cards are the most valuable resource in the game, the player who has more cards is usually more likely to win. That doesn't just refer to drawing more cards (eg. Divination), or making the opponent discard cards (eg. Blightning), but more generally trading one card for two or more of your opponent's (eg. Day of Judgment), creating multiple creatures with one card (eg. Aven Eternal), repeatable abilities (eg. pretty much every planeswalker), etc.
10) Consider the meta game. The meta game is the players and the decks they are playing in your play group. If you know what your opponents are likely to play, prepare your decks accordingly, and play them accordingly. Each meta is different, things that are powerful in one group may be useless in another. So evaluate advice given and cards suggested with your meta in mind.
That's all I can think of right now. But browse around the forums, especially the deck help sections, and you will pick up more pieces of wisdom, I'm sure. Don't give up, trying to get better at playing and deck building is part of the fun of the game.
Some additional thoughts about actually playing the game. You deck is but one part of what wins you the game, how you play it is another. There are some common mistakes that newer players make all the time. Little things, that together add up to make a huge impact. Here are a few of the top of my head:
1) Don't play cards before combat, that won't influence combat. You know what you have in your hand, your opponent doesn't. Attacking with mana open and a card in hand will make the opponent think twice about blocking. Do you have a combat trick or removal in hand to screw him over? Is blocking/attacking worth that risk? Especially in the late game, if you are topdecking, draw a land, and don't need that land, don't play it. You can do that later when you draw another land. You can drop that one land with your land drop of the turn if you need the mana.
2) Creatures die, so don't get too attached to them. With early creatures, the most you can hope for is that is has some effect on the game before it trades with an opponent's card. Don't be afraid of such trades, so long as you don't fall behind on cards by doing so, you ared oing fine.
3) Use your life total a a resource. Especially in the early game, with 20 life at your disposal, you can afford to take a few hits.
4) If your win con includes reducing the opponent's life total, and you can attack unhindered with a utility creature and don't need it otherwise for this turn, attack. It may just be 1 point, but it's 1 point closer to victory. Otoh, if your win con doesn't rely on getting your opponent's life total down, or that tiny bit of damage won't make a difference (like with combo decks), stick to your own plan and don't tempt fate with an attack. There is a lot that can go wrong with attacking (flash creatures, combat only removal, and other combat tricks), so if your creature is essential, don't risk it.
5) Play your cards as late as possible. Open mana and cards in hand is a threat to your opponent. So for example, if you have an instant card draw spell, you may want to play it in the opponent's end step. That way, you use your mana to full effect while keeping the opponent uneasy about what you can do to thwart his plays. another common example is combat tricks. Those you want to use after blockers have been declared.
6) Don't overcommit. If you have a full board already, that one additional creature probably won't change anything in your favor, but a mass removal on your opponent's side will give him even more card advantage. Better to keep the card in reserve to rebuild and keep the opponent guessing.
7) Counterspells are powerful tools, but you only have a limited amount of those. So consider carefully wether your opponent's spell is worth countering. If you can cope with it in some other way already, keep the counter for something more important. Example: your opponent is casting a 3/3 creature, and you have a 0/4 wall out already. No need to counter that creature, since you can block it with the wall. But of course, a creature's ability have to be considered, too. That 3/3 might still be a huge threat due to its abilities.
8) Bluffing is part of the game. I touched on that already a bit with the "keep a card in hand". But you can go to greater heights with that. If your opponent knows that your deck plays a certain card, and you play as if that card is in your hand, you can convince your opponent that you have it in your hand when you don't.
9) Know the cards in the game. You don't have to learn all the cards in the format, but keep in mind the cards that have been played, especially in the current game. This gives you hints about what to expect, and you can adjust your game accordingly. Remember, that you can look at all the cards in all graveyards and in exile at any time. Your opponent is playing his cards according to his plan. If you can figure out his plan, you can throw a wrench in it. So don't just look at what the cards do right now, but what they could do if other cards are around as well. Some cards just scream "combo", long before it is assembled.
I guess I'm just terrible. Lol
1) Have a plan, and have only one plan. Combining strategies or putting in a secondary strategy "just in case" weakens your main strategy as fewer cards are dedicated to it. And the secondary will never be as good as it could be if it were the sole approach, and for the same reason: only part of the deck is dedicated to it.
2) Play full sets of four of your key cards to increase consistency of drawing them. A good deck wants play the same each time as much as possible.
3) Stay at or at least close to the minimum deck size to increase consistency of drawing your key cards.
4) Good decks play good cards. All cards are not created equal, you want to get the most out of every single card slot in the deck. Playing subpar and/or fringe cards just won't cut it in the long run.
5) Play enough lands. Each deck has its own land needs, but most require about 40% of th deck to be lands. That means about 24 lands in 60 cards. Aggro decks can cope with a bit less (though most of those still shouldn't go below 20), Control decks may need up to 28 or even more. (Yes, almost half the deck.) Being too greedy with your lands will hurt the deck much more than the additional spells will help it.
6) Interact with your opponent. You are not playing solitaire (though some combo decks can feel like that), your opponent is trying to win as much as you do. You need ways to hinder his plans, and expect him to hinder you. At the very least, you should have some ways to kill or otherwise neutralize creatures, but discard, counterspells, library manipulation, etc. all fall under this point.
7) Pure life gain is BAD. If you are running cards that simply give you some life and offer nothing more, you are wasting card slots. Gaining life is like patching up wounds while your opponent keeps stabbing you. It's much more efficient to kill the thing that's killing you, or to stay within the metaphor, knock the knife out of his hands. Life gain should alsways come as an additional effect or option, not be the sole purpose of a card. Also, life is a resource, you don't have to protect every single point, only the last one matters. So don't throw valuable creatures in front of attackers if you can take the hit instead.
8) Pay attention to your deck's mana curve. Look up the phrase if you want more details. But in short, the mana curve refers to how many cards you have for each mana cost. The bulk of those should usually be at 3-5 mana with some cards for less mana for the earlier game, and higher cost cards being rarer, since you can't play them early and you don't want them uselessly cluttering your hand. You don't have to play stuff during the early turns, but you need to be able to.
9) Try to get card advantage. Cards are the most valuable resource in the game, the player who has more cards is usually more likely to win. That doesn't just refer to drawing more cards (eg. Divination), or making the opponent discard cards (eg. Blightning), but more generally trading one card for two or more of your opponent's (eg. Day of Judgment), creating multiple creatures with one card (eg. Aven Eternal), repeatable abilities (eg. pretty much every planeswalker), etc.
10) Consider the meta game. The meta game is the players and the decks they are playing in your play group. If you know what your opponents are likely to play, prepare your decks accordingly, and play them accordingly. Each meta is different, things that are powerful in one group may be useless in another. So evaluate advice given and cards suggested with your meta in mind.
That's all I can think of right now. But browse around the forums, especially the deck help sections, and you will pick up more pieces of wisdom, I'm sure. Don't give up, trying to get better at playing and deck building is part of the fun of the game.
Former Rules Advisor
"Everything's better with pirates." - Lodge
(The Gamers: Dorkness Rising)
"Any sufficiently analyzed magic is indistinguishable from science."
(Girl Genius - Fairy Tale Theater Break - Cinderella, end of volume 8)
1) Don't play cards before combat, that won't influence combat. You know what you have in your hand, your opponent doesn't. Attacking with mana open and a card in hand will make the opponent think twice about blocking. Do you have a combat trick or removal in hand to screw him over? Is blocking/attacking worth that risk? Especially in the late game, if you are topdecking, draw a land, and don't need that land, don't play it. You can do that later when you draw another land. You can drop that one land with your land drop of the turn if you need the mana.
2) Creatures die, so don't get too attached to them. With early creatures, the most you can hope for is that is has some effect on the game before it trades with an opponent's card. Don't be afraid of such trades, so long as you don't fall behind on cards by doing so, you ared oing fine.
3) Use your life total a a resource. Especially in the early game, with 20 life at your disposal, you can afford to take a few hits.
4) If your win con includes reducing the opponent's life total, and you can attack unhindered with a utility creature and don't need it otherwise for this turn, attack. It may just be 1 point, but it's 1 point closer to victory. Otoh, if your win con doesn't rely on getting your opponent's life total down, or that tiny bit of damage won't make a difference (like with combo decks), stick to your own plan and don't tempt fate with an attack. There is a lot that can go wrong with attacking (flash creatures, combat only removal, and other combat tricks), so if your creature is essential, don't risk it.
5) Play your cards as late as possible. Open mana and cards in hand is a threat to your opponent. So for example, if you have an instant card draw spell, you may want to play it in the opponent's end step. That way, you use your mana to full effect while keeping the opponent uneasy about what you can do to thwart his plays. another common example is combat tricks. Those you want to use after blockers have been declared.
6) Don't overcommit. If you have a full board already, that one additional creature probably won't change anything in your favor, but a mass removal on your opponent's side will give him even more card advantage. Better to keep the card in reserve to rebuild and keep the opponent guessing.
7) Counterspells are powerful tools, but you only have a limited amount of those. So consider carefully wether your opponent's spell is worth countering. If you can cope with it in some other way already, keep the counter for something more important. Example: your opponent is casting a 3/3 creature, and you have a 0/4 wall out already. No need to counter that creature, since you can block it with the wall. But of course, a creature's ability have to be considered, too. That 3/3 might still be a huge threat due to its abilities.
8) Bluffing is part of the game. I touched on that already a bit with the "keep a card in hand". But you can go to greater heights with that. If your opponent knows that your deck plays a certain card, and you play as if that card is in your hand, you can convince your opponent that you have it in your hand when you don't.
9) Know the cards in the game. You don't have to learn all the cards in the format, but keep in mind the cards that have been played, especially in the current game. This gives you hints about what to expect, and you can adjust your game accordingly. Remember, that you can look at all the cards in all graveyards and in exile at any time. Your opponent is playing his cards according to his plan. If you can figure out his plan, you can throw a wrench in it. So don't just look at what the cards do right now, but what they could do if other cards are around as well. Some cards just scream "combo", long before it is assembled.
Former Rules Advisor
"Everything's better with pirates." - Lodge
(The Gamers: Dorkness Rising)
"Any sufficiently analyzed magic is indistinguishable from science."
(Girl Genius - Fairy Tale Theater Break - Cinderella, end of volume 8)