#1. All are grammatical but they all have different meanings: a) He left NY four years ago. "It has been" would be better than "It is", but the meaning is the same. b) This is a statement about one moment in time. It says nothing about what happened before or after that time (four years ago). c) This is not the same as (a) because (c) leaves open the possibility that he may never have been in NY.

All of these are either grammatical or very close to grammatical, but they are not all equally natural, and some do not mean exactly the same thing. All three usually imply that his last visit to New York was four years ago. But there’s a subtle nuance: * 1a / 1b focus on the time of the visit. * 1c focuses on the period of absence. So the meaning is effectively the same in normal conversation. * 2a = 2b = 2d All mean: * You have eaten nothing during the last ten hours. * 2c tries to express the same idea but sounds awkward and unnatural. It is not grammatically correct.