"He reads you a menu."
why is it in this case, te is used instead of tu?
Ex: "il te lit un menu" instead of "il tu lit un menu"
"tu" is used only when it is subject of the verb. When used as a complement (here indirect complement), you use "te".
Because "lui" is 3rd person singular, while "vous" is 2nd person plural (polite "you" or several persons)