+0
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!
+0
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)
+0
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.
Students: We have free audio pronunciation exercises.
Comments  
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.
 Yankee's reply was promoted to an answer.
Teachers: We supply a list of EFL job vacancies