when的用法：
副词 adv.什么时候,何时
at what time
· When can you come to see me?
你什么时候来看我?
· When did you go to bed last night?
昨晚你什么时候睡觉的?
· When are you going to return it?
你准备什么时候还（书）?
副词 adv.是…时候
at which;on which
· Please tell me when we'll have the meeting.
请告诉我我们什么时候开会.
· I wonder when that was.
我不知道那是什么时候的事.
副词 adv.在…的时候
at or during which
· How I regretted the days when I had played and should have studied!
我多么懊悔我本该好好学习却玩掉了的那么多的日子!
副词 adv.那时
but at that moment
· It happened on a day in November when the weather was very wet and cold.
这事发生在十一月的一天,那时天气又潮湿又寒冷.
· We were about to start,when it began to rain.
我们刚要出发时开始下雨了.
连词 conj.当…时
at the time when
· It happened when I was living in the country.
这事发生在我住在农村的时候.
· It was raining when we arrived.
我们到达时,天正下着雨.
· He always comes on those days when I am busy.
他总在我忙的日子来.
· I was walking along the stream,when I met my teacher of English.
当我沿着小溪散步时遇到了英语老师.
连词 conj.一…就
immediately after
· When I had finished dinner I went to bed.
我吃完饭就睡觉了.
· She was very surprised when she saw me.
她见到我非常惊讶.
· Let us set out when the sun has risen.
太阳一出来,我们就动身.
代词 pron.（关系代词）那时
which time
· I came here in 1949,since when I have been engaged in this work.
我1949年到这里,从那时起我就担任这项工作.
· We came a week ago,since when the weather has been bad.
我们在一星期前来此,从那时起天气就一直不好.
代词 pron.（疑问代词）什么时候
what time
· Since when have you been here?
你从何时起在此地?
· Until when can you stay?
你能呆到什么时候?
if的用法
一、虚拟语气表示一种不能实现的假设.该语法主要用于if条件状语从句.也可用于主语从句、表语从句、宾语从句等.
二、if条件状语从句中虚拟语气的判断
判断是真实条件句还是非真实条件句.只有在非真实条件句中才使用虚拟语气.通过句子意思,看假设的条件是否能够实现,能够实现是真实条件句,不能使用虚拟语气；假设的条件不能实现则是非真实条件句,要用虚拟语气.
判断这个假设是与哪个事实相反.通常有三种情况：①与过去事实相反.②与现在事实相反.③与将来事实可能相反.
三、“后退一步法”
后退一步法是指在准确地判断了该句与哪一事实相反后,按虚拟语气的后退一步法处理从句谓语动词的时态.即：在非真实条件状语从句中,谓语动词按正常情况“后退一步”.也就是：
①与过去事实相反,在从句中用过去完成时形式表示.
②与现在事实相反,在从句中用过去一般时形式表示.
③与将来事实可能相反,在从句中用过去将来时形式表示.
主句中则用情态动词would,should,could 等加一个与从句一致的动词形式.
例：If I had come her yesterday,I would have seen him.
If I were a teacher,I would be strict with my students.
If it should snow tomorrow,they couldn’t go out.
四、注意事项
①if条件句中如有were,should,had,可以省去if,并使用倒装语序.
②在现代英语中if条件状与从句中的谓语动词如果是be其过去形式一般用were.