第21句：

You see,I still have this pain in my back;this medicine the doctor gave me was supposed to make me feel better by now.

六级词汇讲解:
被动语态表示主动含义的用法在英语里很常见，但却与我们中国人的思维习惯相去甚远，考生需多加练习。分号后句子的意思是“医生给我开的药现在应该可以使我感觉好些了(意为说话人的症状现在未见好转)”。

六级考点归纳：
be supposed to的用法如下:
1. 当be supposed to的主语是sb.时，意为“应该;被期望”，可以用来表示劝告、建议、义务、责任等，相当于情态动词should。如:
Teachers are supposed to treat ail students alike。
老师应该对所有学生一视同仁。

2. 当be supposed to的主语是sth.时，意为“本应，本该”，用于表示“某事本应该发生而没有发生”。如:
The train was supposed to arrive an hour ago.
火车本应在一小时之前到达。

3. 当be supposed to后接“have+过去分词”时，表示“本应该做某事而没做，如:
You are supposed to have handed in your homework by now.
到现在你应该已经把作业交上来了。

4. be supposed to的否定结构为be not supposed to，常用于口语中，意为“不被许可;不应当”。如:
She was not supposed to be angry about that.
她本不该为那件事而生气的。