Sentences with introductory It must be distinguished from certain patterns of sentences with impersonal It :
a)With the predicate expressed by verbs to seem, to appear, to happen, to turn out followed by a clause.
It seemed that he did not know the place. ( It- impersonal, describes a certain state of affairs) That he did not know the place seemed
b) With predicative adjectives preceded by too and followed by an infinitive.
It was too late to start (It – impersonal, describes time)
To start was too late. (the infinitive can’t be placed before the predicate)
c) With the predicative expressed by noun time followed by an infinitive.
It was high time to take their departure.( It – impersonal, the infinitive is attribute to the noun time)
To take their departure was time (it can’t be transformed)