Modal verbs' tense consistency

I have a question concerning tense consistency of modals which don't have the past form like (will/would). Do we put these modals in the past tense (perfect) if the surrounding verbs are in the past tense? Or do we leave them in their present form, and we assume, that if the surrounding verbs are in the past form then the modals refer to the past as well even though we leave them in their present form?

Examples:

1. I knew I should do it before. vs. I knew I should have done it before.





2. The doctor said I might be sick. vs. The doctor said I might have been sick.




3. My friend told me I must forget about my ex. vs. My friend told me I must have forgotten about my ex.

I hope you understand what I'm trying to say, many thanks in advance!