in mtg what happens if i cant do all the card says

[Ethan]

in magic what happens if i can't do all what the card says

[Twanos]

Please provide an example... Sometimes you can't play the card, sometimes the spell just fizzles, sometimes the game ends in a draw...

Need to know the specifics to answer.

[Arix]

Here's a basic checklist.

Are you unable to choose all targets (example: Decimate without an enchantment in play)? Then you can't play the spell at all. You need to be able to choose all targets in order to play a spell.

Did you play a spell, then a target became illegal (example: you played Decimate and the targeted creature gained protection from red)? Then the spell resolves and does as much as possible to any targets still legal.

Did you play a spell, then all of its targets became illegal (example: you played Zap and the targeted creature left play)? Then the entire spell is countered and you don't do any of its effects (you don't draw a card in this case).

Is there something that just can't be done (example: you play Innocent Blood when you control no creatures)? Then the spell just does as much as it can (in this case, making everyone who can sacrifice, sacrifice).

Something else? Please specify.