如果不(多用于否定句)，除非……(否则就不)，除非(有下列情况)(except under the circumstances that;except on the condition that)：Come at 8 o'clock/Don't come～I telephone. 除非我打电话(另行通知)，否则请8点到/不要来。I don't think I will write any preface～you are particularly anxious for one. 如果你不是极其需要一个前言的话，我想我是不会写的。U～I'm mistaken， I've seen that man before. 如果没搞错的话，以前我就见过那个人。White would never do this～compelled. 除非迫不得已，否则怀特是不会这样做的。U～compelled to stay in by bad weather， I go for a walk every day. 我每天都去散步，除非天气不好被迫待在家。After April 28th，～redeemed，it'll be for sale. 如若4月28日之前不赎回，东西将被拍卖。

【注意】1)unless和if...not经常通用。但unless不能用于描述未发生事情的条件句中，如：We would have had a lovely holiday if it hadn't rained(*unless it had rained) every day. (如果不是天天下雨的话，我们会度过一个美好的假日。)I'll be sorry if she doesn't come(*unless she comes) to the party. (如果她不来参加晚会，我会感到遗憾。) 2)unless常用来表示一种说明性的想法，即对句中主要内容的一种补充(此时不用if...not这个结构)。如：She hasn't got hobbies—unless you call watching TV a hobby. (她没有什么爱好，除非你把看电视也叫作一种爱好。)Have a cup of tea — unless you'd prefer a cold drink. (请喝茶——如果你不是更喜欢冷饮的话。)