This is the 540th most frequent Vietnamese word.
thay
"Thầy" in Vietnamese means "teacher" or "master" (male).
Here, 'thay' means 'change' or 'replace' something.
Tôi thay áo trước khi đi ra ngoài.
I change clothes before going outside.
In this context, 'thay' refers to 'teacher', used as an abbreviation for 'thầy giáo', meaning a male teacher.
Tôi thấy giáo viên hôm nay đầy năng lượng.
I see that the teacher is full of energy today.
Here, 'thay' means 'substitute' or 'stand in for' someone.