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Mary was home last night when we went to visit her. She may/might have been studying at the library.

May I ask if I could use "could have been studying" instead of "may/might have been studying"?

Thank you for your help!
Comments 
Many people would do that, but it just introduces confusion, so I wouldn't.

may/might shows that you, the reporter, thinks of that as a possible event in the past (might is more doubtful)
can/could shows that the person had the capability of doing it (was able to do)
Sorry, the original sentence should be "Mary wasn't...", not "was".

Mary wasn't home last night when we went to visit her. She may/might have been studying at the library.

Thank you for your answer, Marius Hancu.
Students: We have free audio pronunciation exercises.
Hi Viceidol

I had a feeling that the word 'not' had been omitted.
If the first sentence states that 'Mary was not home', the second sentence makes more sense to me. Emotion: smile

Yes, you can say 'she could/may/might have been studying at the library' to suggest a possible reason that she was not at home.