Jose, If you want to study grammar seriously, it's important to buy a good grammar book. There are also lots of free grammar resources on the internet such as englishpage.com. You've asked a question which needs many pages to give a good answer. I like "English Grammar in Use", by R Murphy. Other teachers may have different preferences.

Thanks so much :D!

I agree that you should look this up on line for a more detailed answer, but I'll give you some sample sentences to help. "I have a problem" "He has a problem" "She had a problem, but now it's solved. " " Does he have a problem? " "Does she have a problem? " " I had a problem once. " So see 'had' is a past tense term, meaning that you possessed something but now you dont. Has is mainly used when referring to some one else. You wouldnt see or say "I has a problem" because it doesn't sound right. Also you wouldn't say 'He have a problem' for the same reason. When you're asking a question,however, you usually use 'have'. "how many apples do you have? " " how much money does Billy have? " "how many bananas did Sarah have?" " how many problems will you have? " As you can see it doesn't matter if the question is asking about the past, present , or future. When in question form you will always use have. hope this helped!