Kenneth G. Wilson (1923–). The Columbia Guide to Standard American English. 1993.
could, might (auxs.)
These auxiliaries can both suggest a possible outcome: She might [could] change her mind, and in many such uses they are interchangeable. Might, however, works with the negative, as in She might come, but then again she might not, whereas, in its “able to” sense, could means something different in the negative: She could resign, but then again she could not makes no sense. Ambiguity can also arise: He might be badly injured and He could be badly injured can both mean either “He may now be injured” or “He may be in danger of being injured in the future.” See also MAY, MIGHT.
