什么时候用there, 什么时候用 it?

这些是固定搭配：

There is something wrong with…..
There is no doubt that ( of )…….
There is no need to do (for)……
There is no hurry about……
There is no(some) difficulty(trouble)in…
There is no help for…….
There is no difference between…
There is no lack of……
There is no possibility that (of)….
There is no room for………
There is no sense in…..
There is no sign of……
There is no probability that(of)….
There is no saying that……
There is no reason why….
There is nothing better than to do
There is a chance that (of)….
There is a possibility that(of)….
There is a hope of……
There seems to be
There used to be
There was a time when….

It's no wonder...
It is the first time(that) I have been here.
It was the first time(that) I had visited the place.
It is (high)time (that) he went to school
It was in the town that I was born.
It is important that we (should) learn it well.
(necessary, impossible, ….)
It is clear that he won’t come today.
It is a pity that you can’t go with us.
It is a wonder that he refused such a good offer.
It was 3 years ago that he joined the army.
It is Li Ping who is in the classroom now.
It was yesterday that I saw him in the street.
It was in the street that I saw him yesterday.
It was not until his mother came back that he went to bed.
It is said that he will come tomorrow.
It seems that he knew all about it.
It seemed that he had known all about it.
It looks as if he had known all about it already.
It is no use (good/useless)arguing with you about it.
It is a waste of time talking to you.
It is very unwise quarreling/to quarrel with him
It is not settled whether I am going to American.
It makes no difference whether I will go there.
It doesn’t matter whether he will come or not.
It doesn’t matter if ( that) he will come with you
It is important (for sb) to do sth(necessary…)
It is time (for sb) to do sth.
It is easy (for sb) to do sth ( difficult, hard….)
It took me three hours to do this.
It’s better to finish it on time.
It would be better to do sth.
It was very kind of her to help me with my English.
It is wise of you to take his advice.
(selfish, unfair, silly, stupid, polite,careless,,,)
It is a long way……

there is/are 代表“有...”的意思，相对来说it的用途比较广些。

比如：there is an envelope (on the desk). （桌上）有个信封
it is an envelope. 这是个信封

错了

世界上只有一个形式主语，就是it

there be 是个句式，表示存在