Many writers wonder if it’s equally correct to use “will” or “would” in an if-clause. The short answer is no, but there are exceptions to the rule. An if- or when-clause (often used to form conditional sentences) generally does not contain “will,” which is the simple future tense of the verb “to be.” 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:
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, but as a modal verb. In other words, “will” is permitted when it is being used to politely express willingness, persistence or a wish. For example, consider the following sentence:
The speaker will only warm up the water if he is excused by the listener.