If I drink too much coffee in the afternoon, I can't fall asleep tonight.

[MichaelLu2000]

"If I drink too much coffee in the afternoon, I can't fall asleep tonight."

Should I use "won't be able to" instead of "can't" in the second part of this sentence?

 

[Barque]

Yes.

 

[MichaelLu2000]

Yes.

Does using "can" change the meaning? I don't know why but "I can't fall asleep tonight if I drink too much coffee in the afternoon" sounds okay.

 

[Barque]

You'd be understood, but it isn't correct.

 

[emsr2d2]

In this context, you'd use "can't" to express a general fact/habit. That doesn't work with "tonight".

If I drink too much coffee in the afternoons, I can't fall asleep at night. (General statement of fact.)
If I drink too much coffee this afternoon, I won't be able to fall asleep tonight. (Statement about a specific event.)

The future tense/voice/aspect of "can" and "can't" are "will be able to" and won't be able to".

 

[Winwin2011]

In this context, you'd use "can't" to express a general fact/habit. That doesn't work with "tonight".

If I drink too much coffee in the afternoons, I can't fall asleep at night. (General statement of fact.)
If I drink too much coffee this afternoon, I won't be able to fall asleep tonight. (Statement about a specific event.)

The future tense/voice/aspect of "can" and "can't" are "will be able to" and won't be able to".

I do not understand why we use “won’t be able to fall asleep tonight.” (Statement about a specific event). Can anybody further explain it?

 

[jutfrank]

I told you in my PM—it's a prediction. If you want to express a degree of confidence about the actuality of a future event, use will/won't.

What explanation do you need?