He did not become a millionaire.
He did not became a millionaire.
Why is became wrong?
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: