The understandable confusion that students are caused by the use of the word 'could' derives from the fact that - reckoned by the lights of traditional grammar - it can function as a form of three different moods of the verb 'can': indicative, subjunctive and conditional. In
When I was young, I could run fast.
(= was able to...) it is past indicative, while in
It would make such a difference if she could help us.
(= were able to...) it is past subjunctive, and in
If I were rich, I could buy a yacht.
(= would be able to) it is present conditional*.
In hypothetical if-clauses, it occurs typically in counterfactual 2nd conditionals, either as a subjunctive form in the protasis denoting an unreal present ability, or as a conditional form in the apodosis denoting an imagined consequence.
It is, of course, perfectly possible for both forms to occur within the same sentence, e.g.
If you could help me, I could finish the job much sooner.
* popular name; more accurately, though, conditional imperfect.
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