This is the 858th most frequent Turkish word.
sen
"You" (singular, informal).
In this sentence, 'sen' is used as the subject pronoun meaning 'you' to address a single person in an informal way.
Sen çok çalışkansın.
You are very hardworking.
Here, 'sen' is used as the subject pronoun meaning 'you', emphasizing the person being addressed in a question.
Bu kitabı sen mi okudun?
Did you read this book?
In this context, 'sen' is used as the subject pronoun indicating the person being addressed, combined with another pronoun 'ben' (I) to form a compound subject.