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[sof]

Why write thank you and not thanks you?

 

[fromatto]

Why write thank you and not thanks you?

Because it's 'I thank you' and not 'I thanks you'.

 

[David L.]

As George M. Cohen would reply:
"I thank you, my sister thanks you, and my parents thank you."

Apart from "I thank you" :
singular person - use 'thanks' - my cousin thanks, my uncle thanks you
persons in the plural -use 'thank' - my cousins thank you, my aunts thank you.