I don't know the correct grammar terms for these but hopefully these examples might help. Firstly "to have" is a simple verb of possession. I (or they) have a book (first person singular or plural present), She has a book (third person singular, present), I (or she) had a book (singular and plural, past tense). Have, has and had can also be used to express actions completed in the past.i.e. "to have done something" I have read that book. She has read that book. When it comes to had, the meaning is slightly different. "I had read that book" sounds funny on its own and requires extra information. For example, I bought a new book from the store but then I realised that I had read it already so I returned it. "had had" occurs in the above situation when taking about the verb "to have" in the past. i.e. I had had a coffee already that morning so I did not need another. Should, would, could are all forms of the conditional tense which can be used with the "to have done something" structure. "should have" is taking about actions that have not been completed in the past. similar to "ought to have" i.e. the student should have studied for the test. This implies that the student did poorly in the test because she did not study. "would have" is talking about actions that were not possible in the past. She would have studied for the test had she not had the flu. I hope those examples help and have not confused you further!