[rewboss]

OK, let's start with "have". This is a verb, which is sometimes a main verb and sometimes a helping verb. When it is a main verb, it usually means "to possess", as in: "I have a dog."

As a helping verb, however, it is used to make the present perfect. This is formed using the verb "have" in the present tense, and the past participle; as in: "I have made a cake."

"Has" is simply the third person singular of "have". The third person singular is "he", "she" or "it"; or "Peter", "Sally", "the man with the wooden leg", "the old woman who lives in a shoe", or whatever. So: "I have made a cake," but "Peter has made a cake."

"Had" is also part of the verb "have". It can be either the simple past tense form, or the past participle.

As a simple past tense, it can mean the same as "possessed": "We had a dog, but it died last year." It can also be used to form the past perfect: "I had made a cake..."

As a past participle, "had" is used to make the present perfect and the past perfect of "have": "I have had some problems," and, "I had had some problems."

The other three verbs are more complicated; they are examples of so-called modal verbs. In their form, they are the past tense forms of "will", "shall" and "can", and you may sometimes see them used that way: "I could do this when I was younger, but now I can't."

However, modal verbs in the past tense are also used to talk about hypothetical situations -- situations that are not real, pure fantasy. This is true of "would" and "could", and sometimes (but not often) "should":

If I ruled the world [but I don't], every day would be the first day of June.
If I had a crystal ball [but I don't], I could see into the future.
If I saw a robbery [but I don't think I ever will], I would/should call the police.

"Should" also has a very special meaning: it indicates important advice.

That wound looks terrible -- you should see a doctor! [You may die if you don't]
If you don't want to go to jail, you should pay your taxes. [It's the law, and I really think you should do this]
You shouldn't bully people. [It's morally wrong]

That's just a very, very brief summary. As riverkid says, it's such a big question...