why we have to use have and had?

Have is used with the first and second person singular present and plural tenses and the third person plural present tense. Again, in plain English, this just means you use have when you are speaking in the first and second person singular and third person plural, i.e., with the pronouns I, you, we, and they. I have a great deal for you. Do you have any money? We have a new ride at the amusement park. They also have a wonderful gift shop.