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:
I knew I should do it before. vs. I knew I should have done it before.
The doctor said I might be sick. vs. The doctor said I might have been sick.
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!
Solution 1:
It will be easier to see the correct patterns of usage if you also look at sentences where the verb of the matrix clause is in the present tense.
The doctor says I might be sick. > The doctor said I might be sick.
The doctor says I might have been sick. > The doctor said I might have been sick.
My friend tells me I must forget about my ex. > My friend told me I must forget about my ex.
You don't use the perfect just because the matrix-clause verb is in the past tense. You use the perfect when the modal verb is describing something that happened/existed before the matrix-clause verb.
This does mean that the word "might" is ambiguous in a sentence like "The doctor said I might be sick": it could correspond in present-tense narration to either "The doctor says I might be sick" or "The doctor says I may be sick".
The use of "might" vs. "might have" is not analogous to the use of "will" vs. "would" in contexts like
- The doctor predicts that I will be sick. The doctor predicted that I would be sick.
There are some contexts where things might get a bit more complicated. In particular, the use of "should have" is I think a bit harder to describe because of its special modal meaning when talking about things like hypothetical situations. There is a WordReference thread about should" vs. "should have" that might be of some use: knew I {should go / should have gone}