-As for ''this'' if you are talking about something singular that is near to you, like ... Look at this pen.
-if that pen is far from you (as if someone holding it and he's away from u) .. i believe you gotta use ''that'' so it'll be like ... Look at that pen.
-when you talk about things in plural which are near to you , you''ll use ''these'' ..Look at these pens.
- if it's far from you , you use ''those'' .. Look at those pens.
- ''their'' i think it's a possessive .
that's all i can tell you , i hope it's useful for you. :)
You use this/ that/ these /those when pointing to things. (Demonstrative adjectives/ pronouns)
This/ that= close/ far Singular object
These/ those= close/ far Plural objects
There is for talking about a place farther away than "here".
Their is a possessive adjective. This is their house.