When should you consider your opponent's legal moves?

a.) Before you commit to a move

b.) After you commit to a move

Next question:

If you were going to write a book on chess that would be for your eyes only, what would you include?

You have to think about your opponent's threats before considering a move.  Then you have to think about possible replies to your move.  A good move accomplishes three things, it meets all the opponent's real threats, advances your plans, and isn't easily refuted.

sai....