Here's why:
* Subject-Verb Agreement: "I" is the subject, and "knew" is the past tense of the verb "know." They agree in number and tense.
* Simple Past Tense: "Knew" is the correct form of the verb "know" in the past tense.
Perhaps you are thinking of a different sentence or are unsure about something else? If you can provide more context or a specific question, I can help you further.