How to use has, have and had in a sentence?

Has, had and have are all verbs. This means they are key elements in a statement. They can however be used as main verbs or auxilliary/linking/helping verbs.
As main verbs;
1. Peter has a car
2. They have all you need
3. We had the opportunity to break it.
Of course, "has" is a singular verb, "have" is a plural verb while "had" is either singular or plural but in past form unlike "has" and "have" that are in present form.
So in the above examples, they function as the main/lexical/acting verb.
However, these same verbs can function as helping verbs whereby they link other main verb in a
sentence. These sets of veebs are called auxilliary verbs as they are primarily there to help the main
verb. Some people categorize them as anomalous auxiliary verbs.
Examples:
1. He has grown so big
2. They have done what they need do.
3. She had apologized before she got the letter.
The three verbs still maintain their subject relationship. If the subject is singular, "has" is used, if plural, "have". If an action had been taken before a recent one, "had" is used for the latter action.

SOURCE