# What is the difference: "doesn't come" vs "won't come"

I was doing an exercise and came across this:

If the teacher ... we ... the exam(not come, not have).

So my guess would be one of the followings:

If the teacher doesn't come we won't have the exam.

If the teacher won't come we won't have the exam.

Could you please explain what should be in places of dots and why?

Generally the rule is that the future (will, won't) can't be used in an IF clause.

IF clauses use the present tense to discuss the condition under which future events will or won't happen. So "If the teacher doesn't come, we won't have the exam" is the correct form.

(Sometimes "will"/"won't" is permitted if the meaning is "is/isn't willing to". If "the teacher won't come" was intended to mean "the teacher refuses to come" then "If the teacher won't come" would be acceptable wording.)

From the British Council website:

We use the first conditional when we talk about real and possible situations.

• I’ll go shopping on the way home if I have time.
• If it’s a nice day tomorrow, we’ll go to the beach.
• If Arsenal win, they’ll be top of the league.

In first conditional sentences, the structure is usually if + present simple and will + infinitive. It’s not important which clause comes first.

It depends on your intent:

1. State the fact that is always true. Then you use present tense in both if clause and main clause. If the teacher doesn't come, we don't have an exam.

2. You want to describe one possible outcome, when the situation is real. You use present tense in the if clause and future tense in the main clause. If the teacher doesn't come, we won't have an exam.

There are other conditional situations that need different grammar, but future tense is not used in the if clause. Take a look here.