to discern (e.g. right from wrong); to discriminate; to distinguish
to know (manners, one's place, etc.); to understand; to bear in mind
礼儀を弁えて行動してください。
Please act with an understanding of propriety.
彼は場の空気を弁えない。
He can't read the room.
子供の頃から分別を弁えた少年だった。
He was a boy who knew right from wrong since childhood.