"One month has four weeks" should be right? एक महीने ...?
एक महीने is the oblique case for एक महीना.
Time to look up oblique cases, i guess. I thought it was mistakenly plural or something!
सप्ताह or हफ्ता? Any hindi natives want to comment on which is the more common usage?
Just in case, like me, you type too fast without checking properly and had typed, "there are four weeks in the month" and then wondered why that was wrong. The होते is what determines that.
Why is it 'mahine' instead of 'mahina'?
It is in oblique case because of the postposition में.