To make the simple past negative, we put did not (didn't) before the infinitive (without to).
He saw the man. He did not see the man.
He went to school. He did not go to school.
He wrote a letter. He did not write a letter.
He had a good time. He didn't have a good time.
The verb "do" (past tense = did) is used as an auxiliary verb there; it is used with another verb in its base/infinitive form. When we place "do/does become" into the past tense, we change "do/does" to "did". We don't change "become" to "became". :wink: