The correct answers are I didn't know the party was starting so early! and I did not realize you were expecting me at six.

When forming a negative sentence in the simple past tense, we use the auxiliary verb did + not followed by the base form of the main verb (e.g., know, realize). Because "did" already shows that the action happened in the past, putting the main verb in the past tense too (like didn't knew) creates an incorrect "double past."