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there is a difference among disappointed or disappointing? when do i have to use the first and when the second one?

and when do I use "to have to" and when "must"?

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miky98there is a difference among disappointed or disappointing?
Is there a difference between 'disappointed' and 'disappointing'?

Yes.

Suppose Alice disappoints Bill.
Then Alice is disappointing and Bill is disappointed.

This works for many other words like this.

Suppose Alice bores Bill.
Then Alice is boring and Bill is bored.

Suppose Alice surprises Bill.
Then Alice is surprising and Bill is surprised.

In each case the person who causes the emotion in the other person is described by the -ing word, and the person who reacts by feeling the emotion is described by the -ed word.

miky98when do I use "to have to" and when "must"?

See Must & have to

CJ