We can use can, could, and may for requests and permissions, but is there any difference between the meanings of the following three versions?
- May I go?
- Can I go?
- Could I go?
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Each of these modal auxiliary verbs has a variety of meanings. While may almost certainly indicates that the speaker is seeking permission, it could also be interpreted as pondering the possibility, so that the first question could be paraphrased as is it possible that I choose to go? That's pretty far fetched though.
The other two are the present and past tense of the same word. Again, the most likely interpretation of these questions is asking permission, but they also readily allow an interpretation of possibility. They may even suggest ability, but this meaning would be more likely if the question was about someone or something else.
The past tense could admits a conditional meaning or may simply be more polite.
One definition of "could" is past tense of "can," but I don't think "could" is past tense here. "Could I go?" clearly refers to future action. See https://en.wiktionary.org/wiki/could