The main thing you need to keep in mind with “have” and “has” is that…you generally use HAVE with (I, you, we, they) and you use HAS with (she, he, it) in the simple present and present perfect forms. You use HAD with (I, you, she, he, it, we, they) in the simple past and past perfect forms. Of course, things get a little more complicated when you start mixing auxiliary and main verb forms of “to have”, so you might need to practice a little more in order to get it right every time.
www.englisch-hilfen.de/en/grammar/have.htm
www.englisch-hilfen.de/en/exercises/tenses/have.htm
www.learnenglishfeelgood.com/g-english-verbs-tohave1.html
www.eslcafe.com/quiz/vtr.html