Present tense vs Past tense

Hi all, I have read a lot of threads about Present tense vs Past tense.

I am still confused, so I would like you guys help me with that.

Below are some examples that made me confused.

1: The grade I received (isn’t/wasn’t) correct. ( It is still not correct, and I want to let the instructor knows.)

Why either one is correct in this case?

Is there any difference between them?

I thought the sentence “The grade I received wasn’t correct” didn’t tell us anything about correctness of the grade now.

So is it better to use “isn’t” if I want to let my instructor knows that my grade is still not correct?

If “isn’t” is better, does it violate the rule “if the first verb is past tense, the second one should be past as well”?

2:The paper he wrote in 1990 (has /had )a lot of mistakes. ( It still has a lot of mistakes, and I wanna tell people about that right now.)

3: The notice he posted on the door (is /was) confusing. ( it’s still confusing. )

Please explain and let me know which option I have to choose for the sentences above.

Please also let me know if my message is confusing. If so, I will try my best to rephrase it.