1. 除非；若不；除非在…的时候
You use unless to introduce the only circumstances in which an event you are mentioning will not take place or in which a statement you are making is not true.
Do not confuse
unless,
except,
except for, and
besides.
Unless is used to introduce the only situation in which something will take place or be true.
In the 1940s, unless she wore gloves a woman was not properly dressed... You must not give compliments unless you mean them. You cannot use
unless to talk about a situation that results from something not happening. Instead you use
if...not.
I'll be glad if she doesn't come. You use
except to introduce the only things, situations, people, or ideas that a statement does not apply to.
All of his body relaxed except his right hand... Travelling was impossible, except in the cool of the morning. You use
except for before something that prevents a statement from completely true.
The classrooms were silent, except for the scratching of pens on paper... I had absolutely no friends except for Tom. You use
besides to introduce extra things in addition to the ones you are mentioning already.
Fruit will give you, besides enjoyment, a source of vitamins. However, note that if you talk about 'the only thing' or 'the only person
besides' a particular person or thing,
besides means the same as 'apart from'.
He was the only person besides Gertrude who talked to Guy. 不要混淆
unless，except，except for 和 besides。
unless用于引出某事发生或成立所必需的条件，如：In the 1940s,
unless she wore gloves a woman was not properly dressed（在20世纪40年代，女性若不戴手套就算衣着不得体），You must not give compliments
unless you mean them（除非出自真心，否则不要随便恭维）。谈论由于某事未发生而导致的情形不能用
unless，要用 if...not，如：I'll be glad if she doesn't come（如果她不来我会很高兴的）。except 用于引出陈述中唯一不适用的事物、情形、人或想法，如：All of his body relaxed except his right hand（除了右手以外，他全身都放松了），Travelling was impossible, except in the cool of the morning （除了在凉爽的早晨，其他时候出行都是不可能的）。except for用于引出使其陈述不能完全成立的事物，如：The classrooms were silent, except for the scratching of pens on paper（除了笔在纸上发出的沙沙声，教室一片寂静），I had absolutely no friends except for Tom（除了汤姆，我一个朋友也没有）。besides用于引出已提到事物之外的其他事物，如：Fruit will give you, besides enjoyment, a source of vitamins（水果不但好吃，而且富含维生素）。但应注意，用于谈论除某人或某物之外唯一的人或物时，besides的意思与apart from相同，如：He was the only person besides Gertrude who talked to Guy（除格特鲁德之外，他是唯一跟盖伊说话的人）。
