【问】 (A) We stayed at the hotel (for) three weeks.
 (B) He taught her for three years.
 许多教科书上说，在for用于表示时间时，在for引导的短语中，for常常可以省略。请问上面两个句子里的for是否都可以省略掉?
 【答】 当for引导的时间短语用来修饰具有静态意义的动词时常常可以省略：
 1. They lived in China (for)seven years.
 2. The meeting lasted (for) two hours.
 3. We stayed at the hotel (for) three weeks.
 4. He has worked here (for) a year.
 5. They will be in Chicago (for) a year.
 当for与以all开头的名词短语连用时，必须省略for[参见for all day(×)]：
 She’s been lying in bed all day.
 但是，当for短语修饰动态动词时或者用于否定句时，则介词for不能省略。
 1. I’m going away for a few days.
 我要离开几天。
 2. I haven’t seen him for three weeks.
 我有三个星期没有看见他了。
 所以，CGEL把下列句子看作是错误的：
 1. × I taught her three years.
 2. × I haven’t spoken to her three months.
 另外，当for用在句子开头时，一般也不能省略：
 1.{He lived abroad (for)six years. For six years，he lived abroad.
 他在国外住了六年。
 2. He waited anxiously at hospital (for) some time.
 For some time，he waited anxiously at hospital.
 他在旅馆焦急地等了一会儿。