Can we then say He may have come yesterday or just He might have come yesterday? Can we use may in the past tense or not?
3 answers
They are pretty much interchangeable, yes.
Ok and the last one. Is there a singular form of sheets? Do we say change your sheets? (sheets on the bed, not a sheet of paper)
Yes, that word has both singular and plural form. You can refer to the top sheet, the bottom sheet, a fitted sheet (the ones whose corners are sewn so that it fits the mattress without having to be tucked in), a flat sheet, a crib sheet (for babies), etc.
http://www.google.com/search?q=fitted+sheet&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-US:official&client=firefox-a
http://www.google.com/search?q=fitted+sheet&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-US:official&client=firefox-a
1 answer