sage said:
Excuse me, this is the first time posting. and hope you could help me, thanks. 
my question is : which one is correct?
1. I just have to stop off in the bathroom.
2. I just have to stop off at the bathroom.
To "stop in" is to stop in(side), whereas to "stop at" is to stop at a location. The goal or end point are exactly the same; the difference has to do with the speaker's intention or view. When s/he says "stop in", s/he's expressing the inside of the location, and when s/he says, "stop at", s/he's expressing the building or room as a whole.
For example: The speaker is inside the bank as she says,
"I am at the bank (meaning, at the bank location)"
"I am in the bank (meaning, inside the bank location)"
If she is standing outside the bank, she could also use "at the bank", but not "in the bank". "at" refers to the structure; "in" refers to the 3 dimentional space within that structure (inside).
Other examples,
"I'll meet you in the park" / "I'll meet you at the park"
"I'll see you at the party", but never: "I'll see you
in the party", because the noun 'party' is not a structure. It has no inside.
All the best,
