This is a tricky question. First, let’s start with the three main parts of the verb “to have.” They are: have, had, had. You use have in the present tense, such as when you say, “I have a headache.” You also use it in the future tense, when you say, “I will have a headache if you keep pestering me.” You use had in the past tense, such as when you say, “I had a headache yesterday.” And you use the third part, which is also had, when you use the three past perfect tenses. I suspect this is more than you bargained for, but I’m getting to “has.” You use “has” only in the third person singular, present tense when you want to say “He has a big car.” Or “She has curly hair.” I suggest you take one of my sentences each day, digest it, and call me in the morning.