In English we can only have one tensed verb in a verb phrase. The auxiliary DO is always tensed. It is always either past tense or present tense.
If we use the auxiliary DO, the Complement verb phrase will always use a PLAIN form of the verb. This is the word you see in the dictionary.
- She didn't PAINT the house.
Notice in the sentence above that because did is in the past tense, the verb paint cannot be past tense. It is in the plain form. Notice also that the verb paint is not present tense. The following sentence is ungrammatical:
- She didn't paints the house. (ungrammatical).