I couldn't tell if he was annoyed or tired.
I couldn't have told if he was annoyed or tired.
What is the difference between these two?
Let me guess, that the sentences are not conditional, for I can substitute if with whether.
The first sentence expresses that I coundn't tell it for some period of time. For example, during our time in college, I did not have the ability to distinguish whether he was annoyed or tired.
The second sentence tells us about one particular time, for example, when he got home two days ago, I coundn't tell if he was annoyed or tired.