Modal verbs are used to express various meanings such as permission, obligation, and necessity. In addition, there are other related verbs that can be used for expressing these meanings such as had better, be able
We use both it and there as a kind of preparatory subject with the real subject placed after the verb. But they are used in different ways and for different purposes.
We use had to to express an obligation or necessity that existed in the past. We use should have to talk about things that didn’t happen in the past but that we wish had happened.
We can use both needn’t and don’t need to to give permission to someone not to do something in the immediate future. Needn’t have, however, means that the action is completed, but it was unnecessary
If you have to do something, it is necessary for you to do it. It’s a law, an obligation or a fact. If you must do something, it is necessary for you to do it,