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Giving permission, we use can or may, though the latter is generally considered more formal : 1 You can/may speak to the patient for just a few minutes (e.g.) now, later, tomorrow. Asking for permissin, we use can or may, asking tentatively ('politely'), we use could or might:
In the reported version of sentence 1, we use could or might : 3 The nurse said we could/might speak to the patient for just a few minutes. We do not, however, use could or might + present infinitive to refer to permission givin in past time. 4 We had (or were given) permission to speak to the patient. Since we were given permission, we presumably acted on it and spoke to the patient. Could and might + perfect infinitive suggest that permission existed but wasn't acted on, perhaps because of ignorance on the past of the subject : 5 You could/might have come yesterday (e.g.) if you had wanted to. |