I think this question is difficult to answer. Why can't it add IS?

Hello, it is the formal object in your sentence.

Explanation: I think (predicate verb), what am I thinking? Followed by an object. At this point, it is an object, but it is not a real object. Because what this sentence wants to express is:

I think this question is not difficult to answer. The real object is to answer this question.

(It just replaces the object)

Help you understand and explain formal objects.

In English, "it" is often used as a formal object rather than an object clause, and the real object clause comes after it, especially in sentences with compound objects.

"it" acts as a formal object, and there are several situations in which the object clause is postpositioned:

1. When verbs such as believe, consider, declare, estimate, imagine, feel, discover, guess, hear, imagine, know, make, prove, recall, think and understand are followed by compound objects (object+object complement), they should be used as formal objects. It cannot be omitted in this structure. For example:

We found it difficult to finish the work on time. I find it difficult for us to finish the work on time.

She thinks it wrong for him not to answer the phone. She thinks it wrong for him not to answer the phone.

Secondly, when a phrasal verb consisting of a verb and a preposition is followed by an object clause, a formal object should be used. For example:

We are considering lending you some money. I'm thinking of lending you money.

Third, it is used in the sentence pattern of "verb+object+prepositional phrase". For example:

Remember that you must be home before ten o'clock. Remember that you must be home before ten o'clock.

Fourth, the case that the object clauses cannot be directly connected.

Verbs expressing "joys and sorrows" such as like, enjoy, love, hate, take, etc. If you want to follow the object clause, you need to follow the formal object it.

I hate it when people talk with their mouths full. I don't like people talking with their mouths full.

Fifth, you can't follow that clause directly.

The following verbs cannot be directly followed by this clause: demand, refuse, make, like, cause, force, condemn, admit, celebrate, beg, dislike, ignore, love, help, take, give, bid, hate, hear, see, want, etc. , but it, facts can be used as a medium, followed by clauses; Or gerund and infinitive. For example:

He ignored that he had made another mistake.

He neglected one thing and made another mistake.

He ignored the fact that he had made another mistake. (correct)

He ignored that he had made another mistake.

I admire them for winning the game.

I admire them for winning the game. I envy them. They won the game.

I admire them for winning the game.

I hope I can help you. If you don't understand, you can ask.