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I am answering an online English grammar test and encountered the following question

Where was Jack yesterday? —I don't know. He ________ seeing the doctor.

My answer is: might has been
Correct Answer is: might have been

Why not might has been? He is singular?

  • "He might have seen", surely, unless he's having an affair with the doctor, or went more than once. – Lee Kowalkowski Mar 30 '11 at 21:11
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Because the verb is "might". After the first verb you use the infinitive, for example:

He has something.

He might have something.

The same rule applies when using "have" to indicate past tense. I will bold the conjugated verbs in the following examples. The following verbs are in infinitive forms.

He is sick.

He has been sick.

He might have been sick.

  • This is actually a pretty bad answer as the modalness of 'might' is the source of the most interesting part of the question, and I didn't address it. Too lazy to fix. – tenfour Mar 30 '11 at 15:42
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Might is an auxiliary (modal) verb (it's actually the past tense of may), and those always require the following verb to be in infinitive form. Other examples of this are can/could and shall/should. (Which is why the (in)famous "I can has cheezburger" strikes us as ungrammatical: the correct form would be "I can have..." – or actually, since it's a question, "Can I have...")