The difference is in the tenses. Had is past perfect, Have is present perfect. For example: I had decided to go to the theatre before he changed my mind. Then we went to the cinema. There is an action in the past "He Changed My Mind", but before this action in the past there is another action in the further past "I had decided to go to the theatre". So the further past action in the past is past perfect. John: So, where shall we go this evening. Have you decided yet? Ann: Yes. I have. I have decided to go to the theatre. In this example there is one action "have decided", and this action does not have a time about when it was completed and also this action was completed at the closest time to now , which makes it present perfect.

Thank you so much for answer :) @shinto and @patrycja

'had decided' is the past perfect tense We use past perfect to describe an action before the simple past. We are speaking about a past before the past 'have decided' is the present perfect tense This also a past action, has a present result or impact. We generally use the present perfect to stress the importance of a past event in the present.

Have decided is present perfect simple, while had decided past perfect, so it’s even more before present perfect simple. I have decided that we’re going on a trip. (General statement, the time is not important) He destroyed my plans that I had decided. (Firstly I made plans for something and then he destroyed it, but both are in the past tense so we use it like that to make it more clear) So it’s even MORE in the past