前者 （假如、一旦）
She glares at me if I go near her desk.
我一走近她的办公桌，她就瞪我。
Do you know if he's married?
他结婚没结婚，你知道吗？
I'm not sure whether to resign or stay on.
我拿不准主意是辞职还是留任。
后者 （除非、）
I sleep with the window open unless its really cold.
我总是开著窗户睡觉, 除非天气非常冷才关上窗户.
I won't go to the party unless I'm invited.
除非我被邀请，否则我不会去参加晚会的。
相反。
比如，
we need to carry an umbrella(伞）with us if it's raining.
we need to carry an unbrella with us unless it's not raining.
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