1. If you play the Ruy Lopez or other classical openings your pawn will either oppose the opponent pawn (so an advance to threaten your knight is not possible), or control the square such that the pawn advance would simply lose the pawn. Openings that do not occupy the center at the start retreat squares are created so the knight has somewhere to go to if threatened.
If your knight is threatened by a pawn, you would usually need to move it away (else you lose material). The main issue is not moving the knight (pieces get threatened and you move or retreat them all the time in any game of chess), but whether if there is anywhere decent to go to. If you have to move the knight to the rim or back to its original square (wasting time) then you either shouldn't have placed the knight there in the first place, or you didn't do a good job protecting the knight from this problem.
2. This gets into a rather complicated analysis and you have to weigh the pros and cons. If you take the knight, obviously your opponent will take back with something, and usually it is to some benefit to the opponent since your opponent has considered this possibility (else the knight wouldn't jump down in the first place). If you retreat instead do you have a good retreat square and is the opponent knight too powerful if it stays there unchallenged?
I just started learning chess and have been playing against the "easy" bot. I have been trying to follow the opening principles, i.e. to develop, the rule of ten, avoid going backwards or to the edges, and following the Ruy Lopez, but often my knights are threatened within the first few moves. Specifically:
1. When the opponent's central pawns advance to attack the newly placed knights, what should I do?
2. When the opponent's knights jump down to threaten my knights, should I drop back or take?
Thank you in advance for your advice.