Being a mathematician, I use LaTeX a lot for writing. However, I am interested (mildly) in working with TeX instead, even if only for keeping my own notes. What is the minimum amount of TeX do I need to know to produce reasonable documents with mathematical content? I'm not interested in the finer points of compositing and typefaces and so on, so I may be barking up the wrong tree here.
Also (and this is a stretch) what about diagrams in TeX? Usually I use xymatrix in LaTeX, is there an easy way to do simple diagrams in TeX?
Edit: The answers to the second question are good, but what about the first? Shall I go back a step and ask:
What is the minimum amount of TeX do I need to know to produce reasonable documents?