This is the 13th most frequent Vietnamese word.
vào
"In" or "enter."
Here, 'vào' is used to indicate entering a specific place, as in 'entering the classroom'.
Thầy vào lớp học lúc 8 giờ sáng.
The teacher enters the classroom at 8 AM.
Here, 'vào' is used to indicate entering a specific location, synonymous with 'inside'.
Chúng tôi sẽ đi vào bên trong tòa nhà.
We will go inside the building.
Here, 'vào' is used to indicate a specific time or season, implying 'during' or 'in' that period.