Which is better, "on" or "in" + place?

Solution 1:

So far as I'm aware, no native speakers would ever say they were on the bathroom.

OP may be getting confused by the fact that BrE speakers can say...

"I'm in the loo" (I'm in the room containing the toilet)
"I'm on the loo" (I'm actually sitting on the toilet)

I don't think there's anything comparable in AmE (except maybe in/on the toilet).

In case anyone thinks it's "odd" that I chose to use I rather than, say, he there, I'll just say that it would normally be considered extremely bad form to say "He's on the loo" (it's always "in" there! ).

Solution 2:

Use in the bathroom. If something is enclosed by walls or bound by some sort of border, use in. You are in a country. You are in the house. You are in college (you are inside the period that starts with enrollment and ends with graduation or dropping out).

On usually means positioned above and physically touching something's top part. So if you said on the bathroom, you are outside sitting/standing on the roof above your bathroom.

The usage of the prepositions in, on, and at are very tricky for those learning English, especially when their native language has one preposition to denote something's location - whether on, in, or at. Unfortunately, they will have to memorize which of in, on, or at goes with which object.

The usage of in, on, and at gets even more tricky because they are also used beyond describing something's location. Phrases such as "on the house" for freebees, "in my career", or "at 3 PM" must be memorized.

When speaking about time, it's still tricky. We say "in 10 minutes", "at the strike of 12", and "on Tuesday". I'm not aware of any rules for the choice of preposition in these cases. As far as I'm concerned, we needed to learn each prepositional phrase individually.