This is the 420th most frequent Persian word.
تو
"You" (singular, informal).
Here, 'تو' is used as the singular second-person pronoun in Persian, equivalent to 'you' in English, addressing someone directly and informally.
تو دوست خوبی هستی.
You are a good friend.
In this sentence, 'تو' is embedded in the possessive pronoun form where it denotes 'your', indicating ownership.
کتاب تو روی میز است.
Your book is on the table.
Here, 'تو' is used as the subject pronoun in a question, addressing the listener directly in an informal manner.