anh also normal, the difference of age between them is not so large, anh is ok
I go for "chú" and "cháu"
Nhất tự vi sư , bán tự vi sư
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1st Version:
Anyone who teaches me one word - even half a word - is still my teacher.
2nd Version:
Even he who teaches me the smallest bit can still be called my teacher
Please correct my grammar as you see fit. Much appreciated!