I recently learned about what commanders where, legendary creatures that go in the command zone before the game starts, so I looked through my cards and added a commander to one of my decks. Earlier today, I was looking to see if commanders could be cast any time an instant could be or only on your turn, then I saw that there was a separate commander format, so now I am unsure about whether or not I can have the commander in the modern style of play.
P.S. The creature I chose was not the main focus of the deck, but was a regular card in the deck, but I made it a commander and replaced it with a different card in the deck itself. By the way, the creature is not part of the commander set.
EDH/Commander is a wholly separate format, all on it's own. It has some special rules, and it's own banlist. I'd recommend checking out the home page for it. The short version: build a 100 card singleton deck out of any cards in Magic (excluding the banlist posted on the site). Your commander must be a legendary creature. You may only use cards that fit the color identity of your commander, I.E. a Blue/White commander deck may only use cards that are blue and/or white. Casting your commander from the command zone uses the same timing restrictions as any other card would. If your commander has flash, cast them whenever you want. Otherwise, you have to stick to your main phases.
There is no Commander in Modern. Modern-legal legendary creature cards can of course be played, but as part of your 60, and not from another zone.
Example: in Commander/EDH you can have Kira, Great Glass-Spinner as your general, or as one of your other 99 providing blue is part of your Commander's color identity.
You can play this card in Modern, but only as part of the 60 in your deck, or as one of the 15 cards in your sideboard. There is no Command Zone in Modern, so you can't cast her from there.
P.S. The creature I chose was not the main focus of the deck, but was a regular card in the deck, but I made it a commander and replaced it with a different card in the deck itself. By the way, the creature is not part of the commander set.
EDH/Commander rule page: http://mtgcommander.net/rules.php
Example: in Commander/EDH you can have Kira, Great Glass-Spinner as your general, or as one of your other 99 providing blue is part of your Commander's color identity.
You can play this card in Modern, but only as part of the 60 in your deck, or as one of the 15 cards in your sideboard. There is no Command Zone in Modern, so you can't cast her from there.
UTeferi, Temporal ArchmageU's prison: blue is the new orange is the new black.
Mizzix Of The Izmagnus : wheels on fire... rolling down the road...
BSidisi, Undead VizierB: Bis zum Erbrechen
GTitiania, Protector Of ArgothG: Protecting Argoth, by blowing it up!
GYisan, The Wanderer BardG: Gradus Ad Elfball.
Duel EDH: Yisan & Titania.
In Progress: Grand Arbiter Augustin IV duel; Grenzo, Dungeon Warden Doomsday.