I am a bit confused, as I know I should not use "could"+verb in the sense of a possibility in the past, such as "He could go there yesterday but he did not". I am well aware I should use past modals here.
One book, however, mentions that could can be used also for "general past possibility", which quite confuses me. I know I can use it for abilities, with senses or mental processes (He thought he could.). What does this general past probability mean? What would be an example of that?
To make it clear, I am not asking about usage of past modals. I am aware of that. I am specifically asking about "could" in situations when it refers to "general possibility in the past".
Best Answer
An example of "general past possibility" would be:
As opposed to specific past possibility:
It is common usage, even if incorrect according to some grammars, to use the same syntax as the general possibility for a specific one:
We're actually seeing different senses of could here. In the present (or future) tense, could indicates a sort of hypothetical, where other languages might use a subjunctive form, perhaps. It is used to make suggestions or raise possibilities.
It doesn't communicate a strong desire, though tone may indicate some desire. It can also indicate a possibility that one might need to think about.
Now, when we want to put a hypothetical in the past, we apply that same sense of could to the present perfect:
Used that way, it is expressing a hypothetical about things having been different. It is known not to have happened, but it could have happened.
That is an unknown hypothetical in the past. You don't know whether it happened or not.
But when we use could in sentence like "in Victorian England, people could die of diseases that we can easily treat today", it's not the same sense of could as above. Let's talk about can.
Can is a modal verb that indicates possibility, rather than a hypothetical. It means be able to, and sometimes you have to resort to be able to because can doesn't exist or work in all tenses. So,
But
But what if you can't touch your toes now, but you were able to in the past? Well, then it becomes:
Here, could is a past form of can. It means, roughly, "was able to". It says nothing about whether you did or not, merely that had that ability.
The two senses of could are closely linked, no doubt. It probably tells someone more educated in philology than I a great deal about how our language developed from its forebears. But now these two sense are quite distinct - if potentially confusing.