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Is it right to use this word when referring to the past:

I didn't knew he was the antecedent of my problem.

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"I didn't know" is a negative Past Simple Tense sentence.
Negative sentences in the Past Simple Tense are formed using did not + infinitive without "to".
"I didn't knew" doesn't look anything to me. It isn't a sentence in the Past Simple, because it violates the rule above.
So, if you want to say something like "I didn't know about the fact some time ago", then be sure did not + infinitive without "to" is completely correct.

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