before的时态用法be doing

　　几个基本时态标志词：

　　一、一般现在时：

　　标志：always, usually, often, sometimes, every week (day, year, month…), once a week, on Sundays,

　　结构：①be动词；②行为动词

　　二、一般过去时：

　　标志：ago, yesterday, the day before yesterday, last week(year, night, month…), in , just now, at the age of 5, one day, long long ago, once upon a time, etc.

　　结构：①be动词；②行为动词

　　三、现在进行时：

　　标志：now, at this time, these days, etc.

　　结构：am/is/are+doing

　　四、过去进行时：

　　标志：at this time yesterday, at that time或以when引导的谓语动词是一般过去时的时间状语等。

　　结构：was/were+doing

　　五、现在完成时：

　　标志：recently, lately, since…for…,in the past few years, etc.

　　结构：have/has + done

　　六、过去完成时

　　标志：before,by,the end of last year(term,month)......

　　结构：had done

　　七、一般将来

　　标志：tomorrow,next day,soon,in a few minutes....

　　结构：1、be going to 2、will/shall do

　　八、过去将来时

　　标志：the next day,the following month

　　结构：1、be going to do 2、would /should do

　　这里列举的都是最基本的，具体题型中还有很多变化。