常常看到 for a year, for three months, for two weeks 等状语中的介词 for 被省略。这当中有哪些规律吗？表示一段时间的介词 for 是有时可以省略还是在任何时候都可以省略？
黎老师讲得非常不错，我补充几个源于《英语疑难问题解答大辞典》的例子.
■当for引导的时间短语用来修饰具备静态意义的动词是常常可以省略。
They lived in China seven years.
The meeting lasted （for）two hours.
They will be in Chicago a year.
■当for与all开头的名词短语连用时，需要省略for.
She’s been lying in bed all day.
■当for短语修饰动态动词时或者用于否定句是，则借此for不可以省略。
I’m going away for a few days.
I haven’t seen him for three weeks.
■当for用在句子开头时，通常也不可以省略。请比较：
a. He lived abroad six years.
For six years, he lived abroad.
b. He waited anxiously at hospital some time.
For some time , he waited anxiously at hospital.
