Use of It

The USE of "It" :

(a) For sentence involving it .... that:

"Two constructions, closely allied, but grammatically distinct, are often confused: 

(i) Antecedent 'it' followed by a defining relative clause with 'that' (who, which); 

(ii) 'it' followed by a clause in apposition, introduced by the conjunction 'that' " – Fowler. 

Thus, in the following examples, the Relative clauses are marked R, and the Conjunction clause, C : 

It is money that I want (R). 

It is you that I gave it to (R). 

It is to you that I gave it (C). 

It was a knife that I cut it with (R). 

It was with a knife that I cut it (C).

"The confusion....ought not to arise, for always with a relative clause, and never with a conjunction, the complement of the main predicate is a noun or the grammatical equivalent of a noun." – Fowler (King's English).

(b) "It" followed by an infinitive phrase — In the following examples, we may analyse It as the formal subject, and the infinitive phrase as the real subject; or we may treat It as the subject, and the infinitive phrase as the real subject; or we may treat It as the subject, and the infinitive phrase as in apposition to "It", and hence, an adjunct to the subject:

It is easy to say so. 

It is hard to believe him.

But in cases like "It is time to go now", the infinitive phrase cannot be treated either as the real subject or as in apposition to It. We must here analyse It as the subject, and the Infinitive as an Adverbial adjunct.

(c) "It" as the formal object: In sentences like –

I found it difficult to do the work. 

I shall see to it that he does his work.

"It is used in a vague sense meaning something, namely. In analysing, we should treat It as the object of the transitive verb or preposition, as the case may be, and regard the Noun Clause or the Infinitive as standing in apposition to it."