This is the 951st most frequent Vietnamese word.
ai
"Who" or "anyone."
In this sentence, 'ai' functions as an interrogative pronoun expressing enquiry about an unknown person or subject.
Ai đã để cuốn sách này ở đây?
Who left this book here?
Here, 'ai' is used as an indefinite pronoun referring to a person not specifically identified.
Tôi không biết ai đã làm điều đó.
I don't know who did that.
In this context, 'ai' as part of 'ai cũng' generalizes to include all people, equivalent to 'everyone'.