" Perhaps you are thinking about situation in which you have the verb "to do" in the question: What did he do? What did happen after that? When you have a form of the verb "to do," it will be conjugated, but the other verb will remain in the bare infinitive form - do/happen/etc.
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in interrogative sentences the verb shouldn’t take the sThat's not true when you question the subject. You may find useful, though it may be much more detailed and complex than you need. Note particularly that if the question word doesn't cross the verb in one of those steps, a do is not introduced and the s stays on the original verb.