"I am teaching in a college."
Translation:Tha mi a' teagasg ann an colaiste.
They are slightly different, although I agree that in both languages there would be quite a lot of overlap
Tha mi a' teagasg ann an colaiste
I am teaching in a college (currently)
Bidh mi a' teagasg ann an colaiste
I will be teaching in a college (in the future) or
I teach in a college (habitual)
The first thing to stress is that there are two different forms of the word an. The one that causes lenition never changes to am.
But there is no hint of lenition here so the rule is that an changes to am before a sound made with the lips (m,b,f,p) as it's easier to say.
This applies regardless of what the word means. It has various meanings, some of which you have not yet met, but it doesn't matter. The rule applies to all the uses that don't cause lenition.