You must needs [needs must] do your duty.
義務をはたさねばならぬ
You must stand and fight or else flee.
立ち上がって戦うか，さもなくば逃げるかだ
You need to understand how this machine works.
君はこの機械の働きを理解しなければならない
You never cease to amaze me.
君には驚かされてばかりだ
You ought not smoke so much.
そんなにタバコを吸うべきではない．
You ought to have told them the truth.
彼らに真実を言っておくべきだった（◆ought to have doneは未来完了を示すこともある：You ought to have gotten a driver's license bef...
You owe it to yourself to ask her for a date.
彼女にデートを申し込みたければ自分でやりなさい
You should wait until you see him next.
次回彼に会うまで待てば
You were stupid to do that.
そんなことしてばかだったね（◆判断の根拠）
You will (just) know.
その時になればわかる
