necessary; needed; essential; necessity; need; requirement
time; hour; period; class
to separate; to part; to divide; to keep apart
to reach the point where ...; to come to ...; to come to be that ...; to start to ...; to begin to ...
exchange; interchange; switching; reciprocity; barter; substitution
promise; agreement; appointment; engagement; convention; rule
state (of affairs); situation; appearance; look(s); sign; indication
how; in what way; how about
boy; bellboy; porter; waiter
older brother and younger sister
to be broken; to break; to break (down); to stop working; to fall through (of a plan, deal, etc.); to collapse