This is the 743rd most frequent Hebrew word.
ידעתי
"I knew" or "I have known."
Here, 'ידעתי' is used in the past to indicate that the speaker was aware of a fact or answer.
ידעתי שזו התשובה הנכונה.
I knew that this was the correct answer.
'ידעתי' in this sentence is negated to convey the speaker's lack of knowledge in the past about someone's arrival.
לא ידעתי שאתה מגיע היום.
I did not know you were coming today.
In this context, 'ידעתי' expresses a strong intuition or feeling the speaker had in the past.