"I didn't went" or "I didn't go"?
I didn't went to the party.
I didn't go to the party.
The rule
After "did" or "didn't," use the base verb, not the past tense. Say "I didn't go," not "I didn't went" — "did" already shows the past.
In negatives and questions, the word "did" carries the past tense — so the main verb goes back to its base form. You say "I went" (past), but "I didn't go," because the past is already inside "did."
More examples
Did you saw it?
Did you see it?
She didn't ate breakfast.
She didn't eat breakfast.
We didn't knew about it.
We didn't know about it.
How to remember it
"did" already means past, so the next verb stays in its base form: didn't go, didn't see, didn't eat.
Frequently asked
Why does the verb go back to its base form?
Because "did" is the past-tense helper, and English only marks the past once. "Did" already carries it, so the main verb returns to its plain form: "Did you go?", "I didn't go."
Is it the same in questions?
Yes. "Did she call?" (not "Did she called?"). The "did" holds the past tense in both questions and negatives.