before
1.从现在说到若干时间以前，不可用before，该用ago，下面两句里的ago都不可改作before：
- I saw him only a few minutes ago.
- He visited us five months ago.
从过去或未来的某时说到若干时间以前，该用before。例如：
- I called at his office yesterday afternoon, but was told that he had gone home half an hour before.
- When you call at his house tomorrow morning, you will probably be told that he has left for the country some days before.
说任何某时（不是现在、过去或将来的某时）的多少时间以前，也该用before。例如：He often tries to use a word or phrase that he has learnt only an hour or two before.
不过从现在说到以前，假使只说“以前”，并不说若干时间以前，便可以用before。例如：
- We (have) met them many times before.
- We (have) never met them before.
注意这两句里的动词属于完成时态或属于过去时态都可以。
2.下面两句的意思不同：
- All day, as three days before, we planted trees. （我们全天种树，正像三天以前一样。）
- All day, as for three days before, we planted trees. （我们全天种树，正像以前三天一样。）
3.比较下面每组里的两句：
- I didn’t go to bed before midnight.(我不曾在半夜以前就寝[在半夜或半夜以后才就寝]。)
I didn't go to bed till(或 until)midnight.(我到半夜才就寝。) - They didn'tmeet before 1953.(他们在 1953年以前不曾相会[在1953年或1953年以后才相会]。)
They didn'tmeet till(或until)1953.(他们在1953年才相会。
4.three hours before里的before是副词，before three o'clock里的before是介词。注意three o'clock是一个时刻（point of time），不是一段时间。before不可在一段时间前面用作介词，所以不可说before a week，before ten years等。不可说I saw him before a week或I shall see him before a week，该改作I saw him a week ago和I shall see him within a week。不可说I shall not see him before a week，该改作I shall not see him for a week。I shall not see him within a week不合习惯。
before long（不久以后）这习语好像和这惯例不合，其实这习语里的before原来是连接词，before long等于before a long time has passed。
5.不可说before a few minutes of the accident或before ten years of his death，该改作a few minutes before the accident和ten years before his death。
注意下面第一句里没有the或for，第二句里有the，第三句里有for。参见after 2：
- A few minutes before the accident she called on me.
- During the few minutes before the accident she had a chat with me.
- For a few minutes before the accident she had a chat with me.
6.在before用作连接词作“…以前”解的时候，它所引导的从句里的动词通常不属于将来时态，如通常不说He will arrive before she will start，而说He will arrive before she starts。但在表示长久以后才会发生的事时往往用将来时态，比较下面两句：
- It will not be long before they come back.
- It will be years before they will come back.
7.before very long（不久以后）比before long表示更长的时间。例如：I hope to hear from him again before long - anyhow, before very long.
8.严格地说，After reading the story, the boy felt, as many others have felt before and since, as if it had been told by himself该改作After reading the story, the boy felt, as many others had felt before and have felt since, as if it had been told by himself。但改后虽然在语法上完美了，读起来却不很自然。
