Why can't I say "could learn"?

Question

I heard I cannot say "I could learn that it is different from **.", and instead, I have to say "I was able to learn that it is different from **."

And after a bit of researching, I saw this explanation:

A. There is one context in which could, with the sense of able to, is not usual. This is when referring to a single past occasion. For example:

? I was late but could get a good seat.

? We were tired but could reach the top of the mountain.

? How many points could you get in the test.

? I could get a bargain.

It is more usual to say was able to ..., succeeded in ..., or managed to ... in such contexts.

There are exceptions to this common avoidance of could when referring to single past occasions.

Exception 1: with verbs of the perception and mental activities:

I could hear a faint noise.

I could understand very little. Could vs be able to

Here, the writer said "with verbs of mental activities", I can use could.

Then why can't I use "I could learn..."?

Answer 1

Could conveys a sense of possibility or a conditional consequence. From the Oxford English Dictionary definition:

1.1 Used to indicate possibility.

1.2 Used in making suggestions or polite requests.

1.3 Used to indicate annoyance because of something that has not been done.

1.4 Used to indicate a strong inclination to do something.

In your statement, if you have already done the learning then it is better to say 'was able to' as this has now happened. If you have not already done the learning then you are be able to say 'could' (I originally wrote you could say could but I thought that might not be helpful!)

So, for example, if you say :

I could learn from this.

you will be understood to mean

There is a possibility that I will be able to learn from this.

However if you have already learned then you will be better understood if you do not say could.