when should we use was and had bcoz they both represent to past .i become confuse to using them

Was - In simple past tense when there is no verb in the given sentence with I, he,she,it,any name,singular subject. Ex:- I was a good student. It's also used in past continuous tense. Ex:- He was going there. Had- In simple past tense as a main verb (v2). Ex:- She had dinner. It's also used in past perfect tense as a helping verb and to show past possession. Ex:- Ram had gone there. Sita had a beautiful watch.