Can I walk between terminals 1 and 2 at SFO or do I have to use air Train? Do I have to reclear TSA?
You can always walk between the terminals outside of the secure area without having to ride AirTrain.
There is a secure connector between Terminal 2 and part of Terminal 1 at SFO that allows you to do so without going through security again. Note that this connector only serves gates 40-48 in Terminal 1 (used by Delta); to the best of my knowledge, you'd need to go through security again to get to gates 20-39.