Questions & Answers about Ti assicuro che tutto è pronto.
Because ti and tu do different jobs.
- tu = you as the subject of the verb
- ti = you as the object of the verb
In Ti assicuro, the subject is understood as I:
- (Io) ti assicuro = I assure you
So ti means you in I assure you, not you assure.
Italian often leaves out subject pronouns like io, because the verb already shows who is doing the action.