When can I say "I could not have passed the test"?

Question

I am nonnative speaker & I often ask a lot of questions that native people have never thought about.

Ok, this site says:

"Couldn't have" is used to show someone's feeling that something in the past is impossible.

Examples:

You couldn't have seen Jerry yesterday. He's been out of town since last Tuesday!

(It's impossible for you to have seen Jerry yesterday because he wasn't here. He's been out of town since last Tuesday!)

So, "someone's feeling" is a guess right? Because if that actually happened, then you use "Couldn't do" right?

Let say I am a very lazy student

Case 1: if I actually failed the test (I got the result in front of my eyes yesterday), I would say: "I couldn't pass the test yesterday" (It was a fact in the past)

Case 2: I took the test yesterday & I will get the result tomorrow, I would say: "I can't pass the test" (I think about the future)

Case 3: I had took the test & the result came out yesterday but I have not checked it (ie I have not officially known I failed), I would say: "I couldn't have passed the test"

I am not sure my thinking is right or not.

But some people said we could say "I couldn't have passed the test" even we have not known the result. That means I did take the exam, I will get the result tomorrow, but I can still say "I couldn't have passed the test" .

Modal verbs are the hardest English stuff I ve ever learned.

Answer 1

I think that you are merging two different uses of "couldn't have" that need to be divided back apart.

1. The idiomatic "Couldn't have" that reflects someone's feeling that something in the past is impossible.
2. The genuine "Couldn't have" or "could not have" that reflects a true impossibility.

The answer to your question, "When can I use 'could not have passed the test'" is that you can always use this construction when the context or the circumstances warrant it. You don't have to worry much about accidentally straying into the territory of the idiomatic usage of "couldn't have" when you don't mean to, and you also don't have to worry about being misunderstood by people as to which of the two usages you're employing at any given time.

Here are some examples labeled with [1] or [2] that will hopefully show the difference between the two kinds.

• I couldn't have passed a physics exam. I'm only in 3rd grade. [1]
• I couldn't have passed the physics exam. I was sick, and then my girlfriend left me, and I never studied, so there was no way I could have done it. [2]
• I couldn't have passed the test. I didn't even show up to take it. [1]
• I could not have passed the test without the help of my tutor. [2]
• I couldn't have passed the driving test. I've never driven a car in my life. [1]
• I could not have passed the driving test. No matter what, that particular driving instructor always gives a failing grade to women drivers. [2]

In a situation where you didn't have the clarifying follow-up sentence (and were simply hearing someone say, "I couldn't have passed the test"), I would typically gravitate toward interpretation [1] for all situations, unless you had strong reasons from the context to believe otherwise.

Also, I would differ from your interpretation on Case 2. I would say that in Case 2, it would be correct to say, "I couldn't have passed the test" and NOT "I can't pass the test." I would only use "I can't pass the test" in a case where the student was speaking before even taking the test.