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  1. She doesn't has a book.
  2. She doesn't have a book.

Why is the first sentence wrong? We use 'has' with singular, and 'she' is singular.

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In a given clause, only one verb is conjugated. When there are auxiliary verbs like "do" combined with main verbs, the auxiliary verb is conjugated and the main verb takes the infinitive form.

In "She has a book," "has" is the only verb and is therefore conjugated: "she has." In "she does have a book," there are two verbs: "do" and "have." Only the first is conjugated: "she does have a book."

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Why is the first sentence wrong? We use 'has' with singular, and 'she' is singular.

Yes certainly. 'She' is third-person singular. The verb 'does' is a singular too. Now what makes the distinction is what our mind interprets. We think the sentence "She has a book" is equivalent to "She does have a book".

This is where the negative comes from. Do and its derived form does are auxiliary verbs used for framing assertive and interrogative sentences. And auxiliary verbs are followed by the raw forms of the verbs, in this case 'have'. 'Has' is not the raw or primary verb.

This is different from the notion of saying "She has a book/books" or "I have a book/books". Here the concept of using 'do' as an auxiliary verb is not present. However, making a negative out of it certainly requires the addition of the auxiliary verb.

Hence we say the following as an answer to the question "Does she have a book?",

She has the book.

She does not have a book. ('She' is the third-person singular.)

And the following as answer to "Do you have a book?"

I have a book.

I do not have a book. ('I' is the first-person singular.)

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