Past Perfect Progressive Tense

Tense : Past Perfect Progressive Tense 过去完成进行式

Formula : verb to have (had) + been + present participle
                  Ex: had been waiting
                        had been watching

Usage :
- An action which has been happening for a period of time in the past and still continues happening at that time
一个在过去的时式中，持续发生着的一个动作，截至使用这个tense时尚未完成那个动作
 
Examples :
a) -I had been eating pizza for two months.
       我持续吃了批萨两个月（到现在我还一直吃，而且还未停，还不知道会吃到几月）

 b) -The couple had been living in that city since their first child born.

Keywords : -since
                     -for

Exercise :

Put the verbs into the correct form (past perfect progressive).

1. We(sleep) had been sleeping for 12 hours when he woke us up.
2. They(wait) ________________ at the station for 90 minutes when the train finally arrived.
3. We(look for) _______________ her ring for two hours and then we found it in the bathroom.
4. I(not / walk) _______________ for a long time, when it suddenly began to rain.
5. How long(learn / she) _______________ English before she went to London?
6. Frank Sinatra caught the flu because he(sing) _______________ in the rain too long.