REPORTING PAST TENSE

Hi everybody

I would like to ask about reporting past clause

Grammatical book write that If the indirect speech is the past tense, then indirect speech will be past perfect

For example:

Direct : I have just written to Jack

Indirect speech: she told me she had just written to Jack

But sometime, i found that we don't need to change the tenses to past perfect. I don't understand when we need to change to past perfect, when we not. Could you please help indicate the rule for this although grammatical book have a n explanation about this.

For example above, if I write an indirect speech like this "She told me she have just written to Jack" ( not change the tense)

Is it correct?

*This is another example, which one is correct?

My father don't know that my mom go to the supermarket. They meet accidentally at that supermarket. When seeing my mother, my father would say to my mother

I thought you (had gone)/ (went) to your friend's house. what are you doing here?

Many thanks in advance.