Is 명 the counter for people?
Can someone explain me when do we use 수무 or 수물?
수물 = 수무 + 를?
We drop the final ㄹ when it comes immediately before a counter. It does not drop otherwise.
When counting, there is no need for the plural particle 들?
Why do I hear 이십 instead of this when saying twenty, from korean people? What's the difference?
이십 is the Sino-Korean way of saying 20. When counting things, its fairly interchangeable with pure Korean numbers.
Why is it "students twenty" rather than "twenty students"?
Its the sentence structure of korean language. Subject+ number+counter+ adjective.