When do you use on in and at for English grammar?

I'm not sure is this answers your question , but this is what I think you were asking.

These are various statements where on, in and at can be used appropriately.

Past tense:

I was in the super market on Monday.

I was at the mall on Friday.

Present tense:

I am on the bus.

I am at work.

I am on my way home.