Conditional sentences. Main clause: “will”, “if” clause: “will”

An “if” or “when” clause generally does not contain “will”, which is also known as the simple future.

One exception is when the action in the “if” or “when” clause takes place after that in the main clause. For example, consider the following sentence:

If aspirin will ease my headache, I will take a couple tonight instead of this horrible medicine.

The action in this sentence is the aspirin easing the headache, which will take place only after the speaker takes them later that night.

Another exception is when “will” is not being used as an auxiliary verb. In other words, “will” is permitted when it is being used modally to express willingness, persistence or a wish. For example, consider the following sentence:

I think I will warm some water for tea if you will excuse me.

The speaker will only warm up the water if they are excused by the listener.