or接一段时间的省略问题
一、当for引导的.时间短语用来修饰具有静态意义的动词是常常可以省略。如：
They lived in China (for) seven years.
The meeting lasted (for) two hours.
They will be in Chicago (for) a year.
二、当for与all开头的名词短语连用时，必须省略for.
She’s been lying in bed all day.
三、当for短语修饰动态动词时或者用于否定句是，则借此for不能省略。
I’m going away for a few days.
I haven’t seen him for three weeks.
四、当for用在句子开头时，一般也不能省略。请比较：
a. He lived abroad (for) six years.
For six years, he lived abroad.
b. He waited anxiously at hospital (for) some time.
For some time, he waited anxiously at hospital.