Difference between Could in the past and Could P.P

Question

How do you understand these two examples?

I could go to Harvard when I graduated from High School.

I could have gone to Harvard when I graduated from High School.

And what is the difference between these two sentences? The preceding two examples are vague to me, but here I don't see any difference in meaning at all.

I couldn't go to Harvard when I graduated from High School.

I couldn't have gone to Harvard when I graduated from High School.

As well as here:

It was so close, I could touch that.

It was so close, I could have touched that.

Correct me if I'm wrong:

Could is used to express an ability to do something in the past, present or future.

Could + P.P. - can be used to say that something would have been possible to accomplish in the past under certain circumstances.

Are they interchangeable? Are there any cases when both mean the same thing?

Thank You!

Answer 1

I could have X usually means that I had the ability or opportunity, but I didn't. I couldn't have X means that I didn't have the ability or opportunity in the first place, so doing X was not an option at all.

I could have gone to Harvard when I graduated from High School.

Feels like: I had the chance, but I chose to do something else.

I couldn't have gone to Harvard when I graduated from High School.

Feels like: Some problem (grades, financial problems, distance, whatever) prevented me from going to Harvard. I didn't have a chance at all.

It was so close, I could have touched that.

Feels like: I could have touched that, but I didn't.

I could go to Harvard when I graduated from High School.

Feels odd. This use of could would be better when speaking of the future: I could go to Harvard when I graduate from high school.

I couldn't go to Harvard when I graduated from High School.
It was so close, I could touch that.

In both cases, I'd be wary of reading more meaning. These are plain statements.