Learn English – How to understand “when to do it” in grammar

grammarphrases

I am confusing of the sentence

I don't know when to do it.

I have opinion of it but I don't know it is right.
I think 'I' is subject, 'don't know' is verb, 'when' is object and 'to do it' is adverb phrase. is it right?
someone said it can be idiomatic expression…. and someone said 

when to do it

is noun phrase which roles object….
What is the answer…?

Best Answer

I: a subject

don't: a contraction of "do (an auxiliary verb) not (an adverb to form the negative)" whose function is negate the sentence (form the negative)

know: a main verb

when (an interrogative adverb) to do (to-infinitive) it (a pronoun): an object of the transitive verb "know".

"When to do it" is a wh-clause which is short for "when I should do it". I would not call it a noun phrase which has a different definition as the link indicates.

As the subject of the sentence "I" is the same subject of the "when clause (when I should do it)", "when I should do it" could be shortened to "when to do it".

You can visit the above link and read more about how the "wh-clause" works.

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