We could hear if anyone came along the trail.

I think it's OK except for the last clause, which should be either "...so quite we would hear if anyone came along the trail" or "...so quiet we could/would hear anyone come along the trail"

But you should know I am not fluent in English, if to put it midly...

"I feel that without 'if' as in your latter suggestion, it's still conditional. What do you think?"

Conditional with "would", but not with "could". Also note that the omission of "if" is not the only change I made: I also replaced "came" with "come". That's essential.

"And I don't see anything wrong with the original, of course, otherwise, I would not have written it :) . What I mean is I don't see 'if' as a reason not to use 'could'."

I don't agree with you, so I think we should ask a more competent person ;)

Ant_222

I also replaced "came" with "come". That's essential.

I noticed that. I would have done that too as hear is followed by a bare infinitive. However, the past tense is probably possible if 'that' is implied.For example, I heard (that) he talked to you.

Ant_222

"I heard he talked to you" — Someone told me he that he had talked to you.
"I heard him talk to you" — You were the direct witness, you heard the talk.

Yes. Exactly. But it doesn't make the original wrong, does it?