면

if, when

Sets up a condition. Korean does not split "if" from "whenever", so the same ending covers a real possibility ("if you listen to the end") and a repeating fact ("when one person stands, the other sits"). It is very often followed by the words for "it's fine" or "you may", which together make the standard way of saying "just do X and you're set". It attaches straight onto the verb stem, so it can be hard to spot as a separate piece.