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..."?