An email from a German friend in Germany had a sentence written . . . "das bin aber nicht ich". Why is "aber" stuck in the middle? How does this fit in the TMP rule? Why "ich" instead of "mich"?

Hi there, the word order in German sentences can change if you want to put more emphasis on a certain word. Here in this case your friend stresses the fact that "this is not him". I don't know the context so it's hard to tell why he stresses that fact. The word "aber" is often placed in sentences like this to stress the "nicht", however in English you would'nt translate it. Your friend is the subject of this sentence, so he refers to himself as "ich". Here are some similar examples where you use "aber" in German. Please note that these are replies to a question or replies to a behaviour: "Das darfst du aber nicht machen" - You are not allowed to do this "Am Montag habe ich aber keine Zeit" - On Monday I don't have time. I hope that helps.