1. am/is - was

例句：I am happy. → I was happy yesterday.

2. are - were

例句：They are students. → They were students last year.

3. beat - beat

例句：He beats the drum. → He beat the drum yesterday.

4. become - became

例句：She becomes a teacher. → She became a teacher last year.

5. begin - began

例句：The game begins at 3 PM. → The game began at 3 PM yesterday.

6. bite - bit

例句：The dog bites the postman. → The dog bit the postman yesterday.

7. blow - blew

例句：The wind blows the leaves away. → The wind blew the leaves away yesterday.

8. break - broke

例句：The glass breaks easily. → The glass broke yesterday.

9. bring - brought

例句：He brings a book every day. → He brought a book yesterday.

10. build - built

例句：They build houses. → They built houses last month.

11. burn - burnt (or burned)

例句：The fire burns the wood. → The fire burnt the wood yesterday.

12. buy - bought

例句：She buys a dress every month. → She bought a dress last month.

13. catch - caught

例句：The cat catches mice. → The cat caught a mouse yesterday.

14. choose - chose

例句：He chooses a book from the shelf. → He chose a book from the shelf yesterday.

15. come - came

例句：He comes to school every day. → He came to school yesterday.

16. cost - cost

例句：The shirt costs $20. → The shirt cost $20 yesterday.

17. cut - cut

例句：She cuts the cake into pieces. → She cut the cake into pieces yesterday.

18. do - did

例句：They do their homework every evening. → They did their homework last night.

19. draw - drew

例句：She draws pictures every day. → She drew pictures yesterday.

20. drink - drank

例句：We drink water every day. → We drank water yesterday.

以上是部分小学阶段需要掌握的不规则动词及其过去式的变化形式。学习这些内容时，建议通过反复练习和实际应用来加深记忆，比如通过造句或者对话的形式。此外，还可以利用一些在线资源和游戏来提高学习的趣味性和效率。记住，语言学习是一个长期积累的过程，持之以恒才能取得良好的效果。