This is the 275th most frequent Latvian word.
tu
"You" (singular, informal).
The word 'tu' in this sentence is a pronoun referring to the second person singular, used to directly address someone in an informal context.
Tu esi gudrs.
You are smart.
Here, 'tu' is used as the subject of the question, addressing a single person informally.
Vai tu mācies latviešu valodu?
Are you learning Latvian?
In this context, 'tu' is used to refer to a second person singular as the subject of the relative clause 'ka tu atnāci.'