Tu versus Ti

I'm confused as to when to use "tu" versus "ti". Would you help me with this? An example would be helpful.

Thanks

Hi William,

Tu is used as a subject, as in:
Tu sei alto. You are tall.
Tu hai tre libri. You have three books.

Ti is used as an object, as in:
Io ti vedo. I see you.
Ti manco una lettera. I send you a letter.
In this case, io is the subject of the sentence.

Hope this helps!

Lucia