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"She must go there".Why is go infinitive?

Neither it is acting like a noun nor it is qualifying any verb,noun or adjective.

If above mentioned qualities are not prerequisites for being an infinitive then why is go in following line not infinitive?

"I go there."

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It is "infinitive" (or "the base form of the verb", as I prefer to regard it) because that is what most modals require.

Only one verb in a verb phrase can be finite, and it is nearly always the first, whether that is a content word or an auxiliary. All the other verbs must be in a non-finite form, either the inifinitive/base form, or (depending on the preceding word) another form such as a participle, or infinitive-with-to.

These are arbitrary facts about English syntax, and are not readily amenable to philosphical argument.

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Supplemental to Colin Fine's answer:

The distinction between the English infinitive and base present forms is usually invisible. In grammatical analysis, however, the distinction must be drawn, because there is one English verb for which the two forms differ: the very common verb BE. Consider your sentences with BE instead of GO:

She must be there. ... She must *are there is unacceptable.
I am there. ... I *be there is unacceptable (though it does occur in some dialects).

We therefore distinguish the base form of other verbs as infinitives when they are used as complements of modal auxiliaries.

Analysis gets a little trickier with situations where the infinitive/base form appears to be employed as a finite verb. Are these uses of be and go infinitives or something else?—

Be honest! Go away!
We demand that he be honest. We demand that he go away.

  • And there's also the conditional use; it's obsolete now, but you're likely to encounter it in older texts:

    If he be honest we will succeed. If he go now we are lost.

I'm inclined to see these as different uses of a single form; but in your studies you will probably be safer following traditional analysis and treating them as distinct 'imperatives' and 'subjunctives'.

  • I don't understand your first paragraph. As far as I am concerned, the base form of "be" is "be". It doesn't make sense to regard any of "am" "is" or "are" as the base form. – Colin Fine Jan 1 '18 at 18:49
  • @ColinFine Note that I said 'base present form'. “By the beginning of the 13th c., the Infinitive and Participle, Imperative, and pres. Subjunctive of am-was, became successively obsolete, the corresponding parts of be taking their place, so the whole verb am-was-be is now commonly called from its infinitive, ‘the verb to be,’ although be is no part of the substantive verb originally, but only a later accretion replacing original parts now lost.”—*OED*, s.v. Be ... Regardless of dictionary headwords I find it clearer to think of the present indicative as the 'base' form. – StoneyB Jan 1 '18 at 19:28

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