While doing an exercise, I came across the following sentence:
If a Carl doesn't come to the party, I'll be really upset.
Shouldn't be it like this?
If a Carl will not come to the party, I'll be really upset.
While doing an exercise, I came across the following sentence:
If a Carl doesn't come to the party, I'll be really upset.
Shouldn't be it like this?
If a Carl will not come to the party, I'll be really upset.
Both sentences are valid, and each will sometimes be the most appropriate choice.
If a Carl will not come to the party, I'll be really upset.
This sentence is about what Carl's perceived or expressed intentions are. "will not come" is speaking of the Carl's current plans about the future. Carl may have the plans right now, so the speaker will be upset as soon as he or she learns of it.
If a Carl doesn't come to the party, I'll be really upset.
This sentence is about the future outcome of Carl's action (or inaction). "doesn't come" is addressing something that Carl might do in the future. It hasn't happened yet, so the speaker won't be upset until it does.
In practice, the distinction comes to down whether one is more concerned about Carl's intentions, or Carl's actions. Sometimes a person intends to do something, but is unable. If "it's the thought that counts", then one would say "If a Carl will not come" because it's important that Carl wants to be there.
If Carl's attendance is what's primarily important (maybe Carl was supposed to help set up the party's buffet), then "If a Carl doesn't come" would be more appropriate.