1. am/was

2. are/were

3. become/became

4. begin/began

5. bring/brought

6. build/built

7. buy/bought

8. can/could

9. catch/caught

这些动词原形在四年级下册的英语教材中可能会出现，建议学生掌握这些基本动词的原形及其用法。