what does this word mean grammaticaly, and how can we make deductions?
If you see that someone drives an expensive car, you might make the deduction that he earns a lot of money. Basically there are three types of deduction.
1) Something is possible: may
e.g. He may speak Spanish - I don't know.
2) There is only one possibility: must
e.g. She must study a lot - she always gets good marks.
3) It's impossible: can't
He can't know me, I've never seen him before.