I did what I [must] [had to] [should have].

[Marika33]

Do all of these work in English or do some of them not work (being ungrammatical)?

1. I did what I must.
2. I did what I had to.
3. I did what I should have.

 

[emsr2d2]

They're all grammatically correct and possible in the right context. These are also possible:

I did what I had to do.
I did what I should.

 

[Tarheel]

I'm supposed to get on UE a couple of hours a day. I do what I have to do and follow the rules.

 

[Marika33]

These are also possible:

1. I did what I had to do.
2. I did what I should.

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".

They're all grammatically correct and possible in the right context. These are also possible:

Could you please give me some context for those sentences so that I'll learn how to decide which one to use?

1. I did what I must (do).
2. I did what I had to (do).
3. I did what I should have (done).
4. I did what I should (do).

 

[jutfrank]

When modal verbs are followed by a past infinitive (have + V3), there's a past meaning. Otherwise, the meaning is present/future.

 

[Marika33]

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?

 

[jutfrank]

I don't think here "I did what I had to do" the meaning is present/future, but it isn't "a past infinitive".

Right. Obviously, the meaning is past obligation. Where 'have to' expresses present/future obligation or necessity, 'had to' expresses past obligation or necessity.

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?

Basically, yes. However, I'd strongly advise you against analysing the semantics of uncontextualised sentences that include modal verbs. If you're going to make up your own examples, make sure you get the meaning clear in your own mind before doing so, and provide enough contextual information to make that meaning clear.

 

[Tarheel]

@Marika33 Do you know why I wouldn't say "I did what I should have done"?

 

[Marika33]

Do you know why I wouldn't say "I did what I should have done"?

I assume it's because "I should've done it" usually implies that I didn't do it.

Basically, yes.

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.

 

[Tarheel]

@Marika33 Yes, if I say I should have done something it means I didn't do it.