1. Both are fine. Although using "people" instead of "persons" is more common in everyday speech. 2. You can say both. 3. This and that are interchangeable in those sentences. Sometimes one will sound more natural than the other. "This" is used to refer to things close and "that" is used for things farther away. If you were looking at a picture of a doctor you could say, "This/that is a doctor" to your friend. If you were at an event with a speaker, you could say the same. But if he were standing right next to you, it would sound weird to use "that". 4. These are the same as 3 except in the past tense, so you could say, "That was a doctor I met in America". You should use "this" if he's close to you. Or if you could say, "Those were doctors in WWII". I hope I understood what you intended to ask!

Your use of the words (That and Those ) is acceptable. In regular conversation, however, that and those are usually used to describe things . So in your accident example, if I was pointing at the doctor (a person), I would say... 'There's a doctor' OR 'He or she is a doctor' . If there is more than one person in a white coat...I would say, 'Are they doctors ? OR "There are doctors over there'. I hope this explanation helps a little.