"Tu" is when you're referring to a close relative, or a friend; it's a personal word.
"Su" is used when speaking with an aquaintance, or peer; it's much more formal.
The word "usted" is also a formal word and means the same thing as "su", however you'd use this when talking to someone older than you.
Lol. "Tu your su" sounds like "tie your shoe" of you're brushing your teeth and trying to talk.
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