2. 将using 改成used。此处过去分词作状语，表示条件。

10.　 Don’t worry about me. I’ll be back on my foot again.

Keys: 1.将seeing改成seen。此处应用过去分词作状语，表示条件。

9.　　　 The first textbooks writing for teaching English as a foreign language came out in the 16^th century.

8.　　　 His old grandpa suffered in heart disease.

7.　　　 The naughty boy constant asked the same question.

6.　　　 He can pass the examination because he keeps study hard.

5.　　　 Some of people could survive the terrible earthquake.

4.　　　 Neither of these creatures are easy to talk to .

3.　　　 Comparing with his brother, he is not so intelligent.

2.　　　 The poison, using in a small quantity, will prove to be a medicine.