if表示条件句，"如果"或"假如"。例如：If winter comes, can spring be far behind? 冬天来了，春天还会远吗？Giving someone all your love is never an assurance that they’ll love you back. Don't expect love in return, just wait for it to grow in their heart; but if it doesn't, be glad it grew in yours. 为某人付出自己全部的爱，并不一定能够保证对方会同样爱你。不要期望爱的回报，就让爱在对方心中慢慢滋长。即使没有，也不要失望，而要为自己心中曾经爱过而感到高兴。If you had stayed, this would never have happened.（虚拟语气）only if表示唯一的前提条件，"只有"。例如：He can continue to support himself and his family only if he produces a surplus. 只有生产有了盈余，他才能够继续养活自己和家人。Only if you put the sun there did the movements of the other planets in the sky make sense. 只有当你把太阳放在中心位置上，天空中其他行星的运动才能说得清楚。On the whole such a conclusion can be drawn with a certain degree of confidence, but only if the child can be assumed to have had the same attitude towards the test as the others with whom he is being compared, and only if he was not punished by lack of relevant information which they possessed. 总的来说，得出这种结论是有一定把握的，但是必须具备两个条件：能够假定这个孩子对测试的态度和与他比较的另一个孩子的态度相同；他也没有因为缺乏别的孩子所具有的相关知识而被罚。if only表示A发生B就会因此发生，"只要"。例如：The senior librarian at the circulation desk promised to get the book for me if only she could remember who last borrowed it. 借阅处的高级管理员答应帮我找到这本书，只要她能记得上次是谁借走的。In the 19th century, British textile makers salivated at the thought of the money to be made if only the empire's representatives could persuade every Chinese to lengthen his shirttail by a foot. 19世纪时，英国的纺织业厂商们盘算，只要大清帝国的官吏们能说服每个中国人把他的衣裾放长一英尺，他们就能够赚多少钱，每念及此，他们就无不垂涎欲滴。The triumphal sermons of Reagan and Thatcher—that the world would be a better place, if only everyone were more like their countrymen—now seem a little hollow and tinny. 里根和撒切尔那些高奏凯歌式的说教，说什么只要人人都更加像他们两人的本国人那样，世界就会更加美好，现在听起来，多少有点空洞黯哑了。if and only if表示"当且仅当"，常用于数学和逻辑学中，可简写为iff（读时仍应逐词读出）。例如：Witches are real if and only if there are criteria for identifying witches. 只有在有标准确认什么是女巫的条件下, 女巫才是真实存在的。An integer n is prime iff the only integers which divide it without remainder are itself and one. 整数n当且仅当它能被本身和1这样的整数整除时才是质数。
