牛津实用英语语法:226 if，even if，whether，unless，but for，
A even if(=even though即便)
请比较下面两个例句：
You must go tomorrow if you are ready.
如果你准备好了，明天你必须走。
You must go tomorrow even if you arent ready.
哪怕你没有准备好，明天你也得走。
B whetheror(=ifor是否)
You must go tomorrow whether you are ready or not.
无论你明天是否准备好了，你都必须走。
C unless+肯定动词(=if+否定动词除非)
Unless you start at once youll be late.相当于：
If you dont start at once youll be late.
除非你立即动身，否则你就会迟到。
Unless you had a permit you couldnt get a job.相当于：
If you hadnt a permit you couldnt get a job.
除非你有执照，否则你就找不到工作。
注意下面两个例句间的区别：
(a) Dont call me if you need help.
如果你需要帮助的话，别打电话找我。
(b) Dont call me unless you need help.
除非你需要帮助否则别打电话找我。
在例句(a)中，即使在对方需要帮助的时候，说话人也不愿意去帮助对方。在例句(b)中，在对方需要帮助的时候，说话人愿意帮助对方，但说话人不愿意对方在不要紧的时候打电话找自己。
unless+hed/youd like/prefer等通常可用来代替 if he/you wouldnt like等：
Ill ask Tom，unless youd prefer me to ask/unless youd rather I asked Bill.
我去问汤姆，除非你宁愿叫我去问比尔。
D but for(=if it were not for/if it hadnt been for不然)
My father pays me fees.But for that I wouldnt be here.
我父亲给我一笔补贴费。不然我就不会在这里了。
The car broke down. But for that we would have been in time.
汽车出毛病了。要不是那个我们会及时赶到的。
E otherwise(=if this doesnt happen/didnt happen/hadnt happened否则)
We must be back before midnight; otherwise well be locked out.
我们必须在午夜之前赶回去，否则我们就会被锁在门外。相当于：
If we are not back by midnight well be locked out.
如果我们不在午夜之前赶回去，我们就会被锁在门外。
Her father pays her fees;otherwise she wouldnt be here.
她的费用是她的父亲付的，否则她就不会在这里。相当于：
If her father didnt pay her fees she wouldnt be here.
要是她的父亲不付她的费用，她就不会在这里了。
I used my calculator; otherwise Id have taken longer.
我用了计算器，不然我花的时间就会更多。相当于：
If I hadnt used my calculator Id have taken longer.
要是我没用计算器，我花的时间就会更多。
F 在日常英语中 or(+else)可以代替 otherwise：
We must be early or(else) we wont get a seat.
我们必须早去，否则就不会有座位了。
G 存在强烈的限制或约束的意思时，provided(that)可以代替if。它主要用来表示允许：
You can camp here provided you leave no mess.
你们如果保持整洁的话，可以在这里宿营。
H suppose/supposing?(=what if?假如)
Suppose the plane is late?相当于：
What if/What will happen if the plane is late?
假如飞机误点的话，将会出现什么情况?
Suppose no one had been there?相当于：
What if no one had been there?
假如没有人去过那里的话，会怎么样?
suppose也可以引出建议：
Suppose you ask him?/Why dont you ask him?
假如你问一问他呢?/你为什么不问一问他呢?
A even if(=even though即便)
请比较下面两个例句：
You must go tomorrow if you are ready.
如果你准备好了，明天你必须走。
You must go tomorrow even if you arent ready.
哪怕你没有准备好，明天你也得走。
B whetheror(=ifor是否)
You must go tomorrow whether you are ready or not.
无论你明天是否准备好了，你都必须走。
C unless+肯定动词(=if+否定动词除非)
Unless you start at once youll be late.相当于：
If you dont start at once youll be late.
除非你立即动身，否则你就会迟到。
Unless you had a permit you couldnt get a job.相当于：
If you hadnt a permit you couldnt get a job.
除非你有执照，否则你就找不到工作。
注意下面两个例句间的区别：
(a) Dont call me if you need help.
如果你需要帮助的话，别打电话找我。
(b) Dont call me unless you need help.
除非你需要帮助否则别打电话找我。
在例句(a)中，即使在对方需要帮助的时候，说话人也不愿意去帮助对方。在例句(b)中，在对方需要帮助的时候，说话人愿意帮助对方，但说话人不愿意对方在不要紧的时候打电话找自己。
unless+hed/youd like/prefer等通常可用来代替 if he/you wouldnt like等：
Ill ask Tom，unless youd prefer me to ask/unless youd rather I asked Bill.
我去问汤姆，除非你宁愿叫我去问比尔。
D but for(=if it were not for/if it hadnt been for不然)
My father pays me fees.But for that I wouldnt be here.
我父亲给我一笔补贴费。不然我就不会在这里了。
The car broke down. But for that we would have been in time.
汽车出毛病了。要不是那个我们会及时赶到的。
E otherwise(=if this doesnt happen/didnt happen/hadnt happened否则)
We must be back before midnight; otherwise well be locked out.
我们必须在午夜之前赶回去，否则我们就会被锁在门外。相当于：
If we are not back by midnight well be locked out.
如果我们不在午夜之前赶回去，我们就会被锁在门外。
Her father pays her fees;otherwise she wouldnt be here.
她的费用是她的父亲付的，否则她就不会在这里。相当于：
If her father didnt pay her fees she wouldnt be here.
要是她的父亲不付她的费用，她就不会在这里了。
I used my calculator; otherwise Id have taken longer.
我用了计算器，不然我花的时间就会更多。相当于：
If I hadnt used my calculator Id have taken longer.
要是我没用计算器，我花的时间就会更多。
F 在日常英语中 or(+else)可以代替 otherwise：
We must be early or(else) we wont get a seat.
我们必须早去，否则就不会有座位了。
G 存在强烈的限制或约束的意思时，provided(that)可以代替if。它主要用来表示允许：
You can camp here provided you leave no mess.
你们如果保持整洁的话，可以在这里宿营。
H suppose/supposing?(=what if?假如)
Suppose the plane is late?相当于：
What if/What will happen if the plane is late?
假如飞机误点的话，将会出现什么情况?
Suppose no one had been there?相当于：
What if no one had been there?
假如没有人去过那里的话，会怎么样?
suppose也可以引出建议：
Suppose you ask him?/Why dont you ask him?
假如你问一问他呢?/你为什么不问一问他呢?
