§ 44. The formal subject expressed by it is found in two patterns of sentences: those with impersonal it and those with introductory it.
1. The formal subject it is impersonal when it is used in sentences describing various states of nature, things in general, characteristics of the environment, or denoting time, distance, other measurements.
Sentences with impersonal it are usually rendered in Russian by means of impersonal (subjectless) sentences.
2. The formal subject it is introductory (anticipatory) if it introduces the notional subject expressed by an infinitive, a gerund, an infinitive/gerundial phrase, a predicative complex, or a clause. The sentence thus contains two subjects: the formal (introductory) subject it and the notional subject, which follows the predicate.
It’s impossible to deny this.
It thrilled her to be invited there.
It gave him a pain in the head to walk.
It was no good coming there again.
It would be wonderful for you to stay with us.
It was lucky that she agreed to undertake the job.
It did not occur to her that the idea was his.
Sentences with introductory it can be transformed into sentences with the notional subject in its usual position before the predicate.
It was impossible to deny this ——> To deny this was impossible.
The difference between the two structural types lies in that the pattern with the introductory subject accentuates the idea expressed by the notional subject, whereas the pattern without it accentuates the idea expressed in the predicate.
Sentences with introductory it must be distinguished from certain patterns of sentences with impersonal it:
a) sentences with the predicate expressed by the verbs to seem, to appear, to happen, to turn out followed by a clause, as in It seemed that he didn’t know the place.
In these sentences describing a certain state of affairs it is impersonal, not introductory and the clause is a predicative one. So it cannot fill the position of the subject:
It seemed that he did not know the place —/—> That he did not know the place seemed. (Transformation is impossible)
b) sentences with predicative adjectives preceded by too and followed by an infinitive as in It was too late to start.
Here it is used in sentences describing time, etc. and is therefore impersonal. The infinitive is an adverbial of consequence, not the subject, and so cannot be placed before the predicate:
It was too late to start —/—> To start was too late.
c) sentences with the predicative expressed by the noun time followed by an infinitive, as in It was high time to take their departure.
In such sentences it is also impersonal, the infinitives being attributes to the noun time. These sentences cannot therefore undergo the transformation which is possible in the case of sentences with introductory it:
It was time to take their departure ―/→ To take their departure was time.
Thus, the subject it may be personal, impersonal, and introductory. In the latter two cases it is formal, (see the scheme after § 45).