This is the 240th most frequent Romanian word.
știu
"I know."
Here, 'știu' is used as a verb meaning 'I know'.
Știu că ai fost ocupat ieri.
I know that you were busy yesterday.
'Știu' is used to express knowledge or skill in performing an action, negated here with 'nu'.
Nu știu să cânt la pian.
I don't know how to play the piano.
In this case, the word 'știi' is the 2nd person singular future tense form of the verb 'a ști', showing its adaptability in tense and subject.