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When speaking about the past, I know that "could" cannot be used for single events (Yesterday I could go there but I did not).
However, what if we talk about an ability that ceased to exist? Is the following correct, idiomatic English?
Yesterday I could lift the couch but today, after the injury, I can barely walk.
All of the answers here are more or less on the right track, but I think are missing some of the details about why..
First, the sentence:
Yesterday I could lift the couch but today, after the injury, I can barely walk.
Is grammatically fine and perfectly idiomatic, but may or may not have the implication you're wanting, depending on what that is (which I'll explain more below).
You are also correct that:
Yesterday I could go there but I did not
is not correct, but I don't think your stated reason is quite right. The problem is not that it's for a single event (that's fine), but that (as others have mentioned) your use of "could" in this way implies that you did, so when you then say that you did not, it is self-contradictory.
The complication here stems from the fact that the word "could", by itself, can be one of two things:
The conditional form of a verb implies that what is being described is not actually taking place, but has the potential to take place, if a particular condition is met. In the above example, the condition is not explicitly stated, but there is an implied condition of:
I could eat that cookie right now (if I decided to).
So how does "could have" fit into all of this? Well, "could have" is simply the past form of the conditional present version of "could". That is, "could have" is the conditional past form of "can". It implies that something in the past did not happen but had the potential to happen if some condition had been met.
So what does this all mean for your original question? Let's look at the different possible forms of lifting the couch:
I can lift the couch. (now)
This is the simple present. You are simply expressing that you have the ability to lift the couch. There are no particular implications beyond that, however.
I could lift the couch. (now)
This is the conditional present. You are expressing that you would be able to lift the couch if you wanted/decided to, but you might not.
Yesterday, I could lift the couch.
This is the simple past. You are expressing that you were able to lift the couch, and implying that you actually did.
Yesterday, I could have lifted the couch.
This is the conditional past. You are expressing that you were able to lift the couch, but implying that you did not (because you did not decide to, or something else prevented you from doing so).
I hope this helps clear things up a bit..
I'm not sure if lifting the couch is an ability, to me it seems more like a single event, lifting a specific couch, that occurred yesterday. But if you lift couches on a regular basis because you work for a moving company for example, it is a general ability and you could say something like: "Before the injury I could lift couches, but now I can hardly walk."
You got that switched up.
If you had the option, but didn't use it, then the assumption must be that something hindered you, perhaps your own will. In essence, doing something you prove that it can be done. As @Sherwood said, if you could lift the couch, yesterday this implies that you did.
Otherwise we say could have lifted whereby could is in the present (for sake of the argument) and modal over have. I could lift the couch, now, I could have an argument now (that is I'd like to, because why would I mention it if I do not in fact want one), I have lifted and I could have lifted. However, while could works as a subjunctive, it doubles as the preterite of can. I can lift, I could lift, but only when there's no ambiguity. This is because the preterite and conjunctive inflection fell together in English, for some reason, where German e.g. makes a difference between preterit konnte and conjunctive könnte. Effectively, could has become an individual verb form on it's own that has no preterite. And that's why yesterday I could is an oxymoren, like tomorrow I was.
Your sentence is idiomatic and absolutely fine.
In addition to its role in expressing conditional possibilities in hypothetical situations, "could [verb]" is also the past tense of "can [verb]", irrespective of whether that verb actually happened or not. So:
"Yesterday I could lift the couch, but today, after the injury, I can barely walk."
is grammatical and makes sense. ... Yesterday I could; today I can't.
"Last month I could lift that couch, but yesterday I couldn't even lift that little footstool."
Last month I could. Yesterday I could not.
Options proposed by others are also OK:
1) "Before the injury I could lift couches, but now I can hardly walk."
2) "Yesterday I could have lifted the couch but today, after the injury, I can barely walk."
