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For example- Does she look smart? Did she come yesterday?

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  • What else would you use?
    – James K
    May 1 '20 at 8:30
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    The dummy auxiliary verb "do" that is used in negation and questions requires a complement with a plain (base) form of the verb. The tense of the clause is conveyed by the auxiliary, either "do", "does" or "did".
    – BillJ
    May 1 '20 at 8:38
  • @BillJ, The OP knows that; the question is why. I suggest that this question has no answer.
    – James K
    May 1 '20 at 8:43
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    The answer is that it's rule of grammar, and sometimes 'rules' cannot be explained.
    – BillJ
    May 1 '20 at 10:14
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    Grammatically, we could add that two consecutive primary (tensed) verb forms are not permitted. The verb preceding the subject is always a primary form, preterite or present tense, and the verb must be an auxiliary with do added if this requirement would not otherwise be met. Why do we have such a rule? Who knows?
    – BillJ
    May 1 '20 at 10:16
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That's because the most important aspect of a verb (or any other distinct meaningful element of speech, for that matter) is its semantics—what it actually means. The base form of a verb (its most rudimentary form) has got exactly that and nothing else. Other aspects of a verb, such as its tense and mood, are taken care of by the auxiliary verb—in your case, does and did.

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  • But we don't say I have eat, or I am eat
    – James K
    May 1 '20 at 8:29
  • Right. All that I said in my answer only applies to the examples provided by the OP in the question. May 1 '20 at 8:30
  • Which is why I don't think it really answers the question. In fact I don't think that the question has an answer. We could use a participle and or we could agree in tense and number with the auxiliary and it would be as logical. It just wouldn't be English.
    – James K
    May 1 '20 at 8:36
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    Maybe you're right. I would think that the OP was wondering why we don't say "does she looks smart?". And my idea is that the "s" at the end of "looks" is indicative of the tense, but that task is assigned to the auxiliary verb. So, the auxiliary steals everything from the main verb except for one thing that it can't steal: the semantics. May 1 '20 at 8:42

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