条件状语从句是表示主句动作发生的前提或条件的从句.条件状语从句分为真实条件状语从句和非真实条件状语从句.引导条件状语从句的有if, unless, so as long as, as so far as, on condition that, in case, suppose, supposing等.条件状语从句中的谓语动词一般要用现在时或过去时代替一般将来时或过去将来时. A．if if表示正面条件.意为如果. If you ask him, he will help you. 如果你向他请求.他会帮助你. Difficulties are nothing if we are not afraid of them. 如果我们不怕困难.困难就算不了什么了. If I do not understand what he says, I always ask him. 我不懂他的话时, 总是去问他. 比较: if only和only if的对比. if only 解释但愿.要是??就好了.表示一个不可能实现愿望.要用虚拟语气. only if 解释只有.等于only on condition that.从句用陈述语气. Only if you heat ice, it turns to water. 只有当你给冰加热.它才会变成水. If only I knew! 要是我知道该多好. B．unless unless = if not, 表示反面条件.意思是如果不.除非. They will go tomorrow unless it rains. 除非明天下雨.否则他们会去的. (They will go tomorrow if it doesn't rain.) I won't let you in unless you show me your pass. 如果你不出示通行证.我就不让你进来. (= I won't let you in if you don't show me your pass. ) 注意: if...not和unless通常是可以换用的.但在下列情况下.两者是有区别的:1．unless多引导真实条件句.if-not可以引导真实条件句或非真实条件句. He won't be able to pass the final exams unless he works hard. 除非他努力.否则就通过不了期末考试. He won't be able to pass the final exams if he doesn't work hard. 如果他不努力.就通过不了期末考试. He would pass the final exams if he worked hard. 要是他努力的话.他就会通过考试.(非真实条件句.含义是He doesn't work hard.) 【查看更多】
