+0
Dear teachers,

Could you please correct me?

We express ability in the past by using "Could+ Infinitive", and in the present by using "Can+ Infinitive". So the past of can is could without the perfect infinitive.

We express possibility in the past by using " Could+ Perfect Infinitive", and in the present by using "Could+ Infinitive".

Thanks
+1
It's difficult to respond without concrete examples. I give a few below with very simplified glosses. Do these help?

I can see you tomorrow. (real possibility/ability)
I could see you tomorrow. (less real possibility/ability)
* I could see you yesterday. (incorrect)
I can play the piano. (inherent ability)
I could play the piano when I was five. (inherent ability)
He speaks English fluently. He can't be Russian. (logical deduction)
He speaks English fluently. he couldn't be Russian. (less certain logical deduction)

I'll stop there. More later if these help.
Comments  
Sounds correct to me.
Teachers: We supply a list of EFL job vacancies
angel girl1We express ability in the past by using "Could+ Infinitive", and in the present by using "Can+ Infinitive". So the past of can is could without the perfect infinitive.
The "could " usage sometimes is confusing. As you described, " could you pass the salt please ? " is not a past tense context but a present usage equivalent to " The salt shaker is too far for me to reach, please help pass it down if you don't mind". In some present contexts, "could " may infer possibility.

The past context inferring possibility that comes to mind would be: I could have finished the project if not for your interruption.
Thanks a lot.
I'm talking about using expressing ability and possibility.
I think The first examples express requesting, and the second example expresses logical deduction.
 fivejedjon's reply was promoted to an answer.
Students: Are you brave enough to let our tutors analyse your pronunciation?
Thanks a lot for your help.