What part of speech is "know" in:

Let us know.

"You" is the implied subject, "let" is the verb, and "us" is the indirect object.

But I'm confused about "know" - what is its grammatical function in this sentence?

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  • Edit the question and let us know :-) what's confusing you about it. – Matt Gutting May 1 '15 at 14:08
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    The mistake is thinking that us is the indirect object. It's not; let is not a three-place predicate so it can't have an indirect object. Us is the subject of the infinitive clause that know is the verb of, and that clause is the direct object of let. – John Lawler May 1 '15 at 14:52
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    @JohnLawler Well, this sense/use of let isn’t a three-place predicate. Other senses/uses can be, such as “He let me the flat at a very favourable price”, where me is an actual indirect object. – Janus Bahs Jacquet May 1 '15 at 15:45

us + know is the infinitive complex object of the sentence.

Infinitive complex object explained, with examples on English Grammar

The combination of a noun in the common case or a pronoun in the objective case and an infinitive used after the predicate forms a complex object. The relation between the noun (pronoun) and the infinitive is that of subject and predicate.

Key examples:

  • I saw the boy raise his hand.
  • I heard him call my name.
  • I want you to know that it doesn't matter.

If you doubt about the object, just think what questions the sentence answers.

The questions are

  • Let us what? (if someone did not hear well)
  • Let us know what? (if someone is not sure what info to give)

Other forms of the same questions may be:

  • Will you let us know?
  • Would you let us know?

'Know' is a verb in the bare infinitive form used after a pseudo-modal verb 'let'.

The rule is that verbs following modal verbs and some verbs (let, make, see, hear, feel, work, help) are used without 'to' particle, in bare infinitive form.

Let us know = [You] let us [to] know.

See other examples on 'let' on Cambridge Dictionaries Online

Let them walk home on their own. (third person)

Let there be no doubt about it. (impersonal)

Let’s not argue about money. We can share the costs.

Don’t let’s throw away the good books with the damaged ones. We can sell them.

  • 1
    Not quite. The infinitive clause (for) us (to) know is the actual object of let, a verb that allows B-Raising (whence let there be...), and which also requires infinitives to delete both for (the subject infinitive complementizer, usually deleted, even when a subject NP is present, as here), and to (the verbal infinitive complementizer, deletable only by special dispensation, as here). – John Lawler May 1 '15 at 14:49

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