'Have' is present tense. 'Had' is past tense. It's also the past participle. If you have any questions about the use of these words in specific sentences, please give some examples, and we'll do our best to help.

I'm answering your second question here, rather than in the 'comments' box, because it's easier to give long answers here. You've asked a very big question, but I'll give you the most common uses. 'Shall' and 'should' are both used in British English with the first person (I/we) in questions when you are making offers and suggestions. eg: Shall we go for a walk? (a suggestion) Shall I lay the table? (an offer) You can also use 'should' in these questions: Should we go for a walk? (a suggestion) Should I lay the table? (an offer) They basically have the same meaning as the ones above, but they are a little more 'tentative', which means that you are saying 'Do you think it's a good idea if ...' 'Should' and the opposite 'shouldn't' is also used when you are giving advice, or saying what you think is the right or good thing to do. eg If you feel ill you should go to the doctor's. Do you think I should buy this new car? Children should show respect to their parents. 'can' means 'it is possible' I can speak English. Birds can fly. She can't play the guitar. Can you help me? You can't be serious! 'could' is the past of can: He could swim when he was three years old. You couldn't fly non-stop to America in the 1920s. However, there are some situations where 'could' is used when it isn't a past tense. In these situations, 'could' is an alternative to 'can', but (like the difference between 'shall' and should') the form could is more 'tentative'. This means that you are less sure. eg Could you tell me where the station is?

"Have" is the present tense- something you have at the moment, so you would say "I have blue eyes." "Had" is the past, so you would say, "I had a good time." Hope that helps!