First time asking a question, sorry for any weirdness. The best way for me to illustrate might be with some examples. I believe all 4 of the following are both grammatical and would be commonly used in English.
- This is the place I was talking about.
- That is the organization I am a part of.
- This is the problem I was having trouble with.
- That is exactly what I was hoping for.
Are the objects of the preposition place/organization/problem/what? Or this/that/this/that? Is there a good rule of thumb to figure it out? In my experience, you can usually make a question of a prepositional phrase and answer it with the object, like "I went onto the roof" can be "where did I go onto?" "the roof" or "I ran into the store" can be "where did I run into?" "the store". But the answer to "what was I having trouble with?" could be "this" or "(this) problem". And the answer to "what was I hoping for?" is "that". Does the last one even have a prepositional phrase?