Why is this sentence correct: "Could you find Diana's bag?" According to what grammars say, it should be: "Did you manage to find Diana's bag?" Because: 1.it indicates ability in a particular moment. 2."find" is not a perception verb. 3.there is no negative limiting adverb. 4.it does not occur in a subordinate clause. I found this example in an activity and it marks as wrong the second example but not the first one,but I still can't understand why.
"Could you find Diana's bag?" and "Did you manage to find Diana's bag?" are both grammatically correct but mean (at least) 2 different things. Unfortunately, "could" is a difficult word to master because it has so many different meanings and usages, and a lot of the differences are subtle.
"Could you find Diana's bag?" is the colloquial/idomatic form of a request to find the bag (i.e. "Please find Diana's bag") and how most native US English speakers would interpret the meaning of that sentence, though some people find that usage irritating because of its literal meaning, which is "Is finding Diana's bag a task you are able to accomplish?" Those people would prefer to be asked "Would you find Diana's bag?"
In the literal case, "could" is the past tense of "can" but is not being used as a past tense but rather, as is true with many other modal verbs, the past tense is used to indicate some tentativeness or probability in the present. People can and do as "Can you find Diana's bag?" but the definiteness of "can" makes that request more forceful and therefore impolite for the people who accept the idiomatic sense of the word, and it really irritates the people who focus on the literal meaning because by forcefully asking "can you do something" the speaker is implying that they (the speaker) believe you (being spoken to) cannot do it. With "could", they are at least saying they think it is reasonably possible or perhaps even likely that you can.
"Did you manage to find Diana's bag?" assumes that you previously indicated that you would look for it but somehow expressed uncertainty about whether or not you would be successful, and the speaker is now asking for an update on that later activity.
could you find can refer to a general state of affairs in the past:
When you lived on the tundra in Alaska, could you find enough food to eat?
However, your example with "Diana" and the search for her bag, if it is not a request (i.e. Could you please find ...) would seem to refer to a discrete incident in the past, since we don't generally look for Diana's bag as a habitual or regular practice:
Could you find Diana's bag? unidiomatic when not a request
and you are correct that the usual ways to ask a question about a particular incident in which someone was searching for something:
Were you able to find it?
Did you manage to find it?
Did you find it?
That isn't to say that we couldn't concoct a scenario where the question was idiomatic; we'd just have to make the search something regular:
When we were first going out, Diana refused to believe I had ESP, and she would test me by hiding her bag and then demanding I find it.
-- Well, could you find it without ransacking the place?