I used to think pm/am was correct, but at some point, i switched to using p.m./a.m. In the context of some kind of dispute, as in. I know that in practical, casual writing, people tend to use whatever form is most.
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I am on it in your first example sounds like a shortened version of i’m on the case, a colloquial way of saying that the speaker is dealing with it.
For reasons i can't recall.
