Here’s the two sentences:

This has not always been the case.

This might not have always been the case.

Obviously, using "has" in the first and "have" in the second is correct. I know that. But, what is the rule that says that is it the case? The only different between the two is “might not” but I am just curious as to what rule established this usage.


It is categorized as Modal verbs. If you are hesitated to use either has or have, Modal verbs are always follow by have. You can find more information about its usage with "have" here.

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