When the subject is a clause, the sentence usually begins with it. So instead of saying ‘That he was once a communist is true’, we say, ‘It is true that he was once a communist’.

It does not matter whether he comes or not. (Whether he comes or not does not matter.)
It is required that he should pay the fine. (That he should pay the fine is required.)
It is clear that he overheard our conversation. (That he overheard our conversation is clear.)
It is clear that you are not interested in this offer. (That you are not interested in the offer is clear.)
It cannot be denied that they tried their best to help him. (That they tried their best to help him cannot be denied. )
It doesn’t matter whether we buy it now or later. (Whether we buy it now or later does not matter. )

Introductory it with seem, appear and look

Introductory it is also used with seem, appear and look when the subject is an infinitive phrase, a phrase with a gerund in it or a clause.

It looked doubtful whether she would come
It seemed strange that she should behave like that.
It seems possible that she may quit the job.
It appeared unwise to offend him.
It does not seem much good going on with the work.

Introductory it as an object

It is sometimes used as the object of the verbs think, feel, deem, count, consider etc.

Don’t you think it dangerous to drive so carelessly?
I consider it a privilege to have this opportunity of welcoming you.
I think it odd that she doesn’t write to me these days.
I think it a pity that she could not win.
We think it improper that he should be so dictatorial.
He made it clear what he wanted.
I find it difficult to talk to him.

Introductory it in questions

The introductory it is sometimes used in questions.

Who was it that broke the window?
It is Peter who broke the window.
Why was it that he stole the bread?
It was because he was poor that he stole the bread.
When was it that the manager came?
It was at 10 am that the manager came.

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)

It is easy to learn English. (= To learn English is easy.)
It is easy to find fault with others. (= To find fault with others is easy.)
It is difficult to know his motive. (= To know his motive is difficult.)
It is difficult to find a good job during these troubled times.
It is dangerous to play with fire.
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.

To err is human. (OR It is human to err.)
To become a well known writer was his life-long ambition. (OR It was his lifelong ambition to become a well known writer.)
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.’

It won’t be any good complaining to the manager. (Complaining to the manager won’t be any good.)
It is silly throwing away this opportunity. (Throwing away this opportunity is silly.)
Will it be any good my talking to him about it? (Will my talking to him about it be any good?)
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?
Many of these sentences can also be re-written as exclamatory sentences.
How silly of you to throw away this opportunity!