did

1.用于有行为动词的一般过去时中构成否定句、一般疑问句和附加疑问句中。

如：He didn't go to school yesterday. ？（注意：用了did后，动词要用原形）

Did he go to school yesterday？（注意：用了did后，动词要用原形）

He went to school yesterday, didn't he ?

2.用于强调一般过去时的谓语。如：He did go to school yesterday.（他昨天确实去上学了）

was

1. 用于构成过去进行时：was + ving. He was doing his homework at that time.

2.做系动词，意为： 是， 表 过去是... She was a teacher .