When the subjective is an infinitive phrase
We begin a sentence with it when the real subject is an infinitive phrase. So instead of saying, ‘To accept your advice is difficult’, we say, ‘It is difficult to accept your advice’.

Structure: It + verb + subject complement + infinitive phrase (real subject)
Ø It is easy to learn English. (= To learn English is easy.)
Ø It is easy to find fault with others. (= To find fault with others is easy.)
Ø It is difficult to know his motive. (= To know his motive is difficult.)
Ø It is difficult to find a good job during these troubled times.
Ø It is dangerous to play with fire.
Ø It could be dangerous to drive so fast.

Note that when we wish to emphasize the infinitive phrase, it may be put at the beginning, especially when it is short.
Ø To err is human. (OR It is human to err.)
Ø To become a well known writer was his life-long ambition. (OR It was his lifelong ambition to become a well known writer.)
Ø To invest all your money in shares is foolish. (OR It is foolish to invest all your money in shares.)


When the subject is a gerund phrase
When the real subject is a phrase that includes a gerund, it is used as a provisional subject to begin the sentence. So instead of saying ‘Your trying to fool us is no good’, we say, ‘It is no good your trying to fool us.’
Ø It won’t be any good complaining to the manager. (Complaining to the manager won’t be any good.)
Ø It is silly throwing away this opportunity. (Throwing away this opportunity is silly.)
Ø Will it be any good my talking to him about it? (Will my talking to him about it be any good)
Ø It is no fun having so many children to look after. (Having so many children to look after is no fun.)

Note that it is possible to change the gerund into an infinitive.
It won’t be any good for me to complain to the manager.
It is silly (for you) to throw away this opportunity.
Will it be any good for me to talk to him about it?