As I understand, the first one is just a full version of "I did what I had to", while the second one is a new one, the full version of which is "I did what I should do".These are also possible:
1. I did what I had to do.
2. I did what I should.
Could you please give me some context for those sentences so that I'll learn how to decide which one to use?They're all grammatically correct and possible in the right context. These are also possible:
I don't think here "I did what I had to do" the meaning is present/future, but it isn't "a past infinitive".When modal verbs are followed by a past infinitive (have + V3), there's a past meaning. Otherwise, the meaning is present/future.
I don't think here "I did what I had to do" the meaning is present/future, but it isn't "a past infinitive".
From these words I can conclude that the difference between sentence 3 and 4 is "I did what I should (usually) do" and "I did what I should have done (at that moment in the past)", is my understanding correct?
I assume it's because "I should've done it" usually implies that I didn't do it.Do you know why I wouldn't say "I did what I should have done"?
I found this sentence "I did what I must" in a movie, and I paid extra attention to this phrase, because I thought we should use "had to" instead of "must" in the past.Basically, yes.