Can ti and tu be used interchangeably?

No, "ti" and "tu" are not interchangeable in Spanish. They are different forms of the second-person singular pronoun, and their use depends on the level of formality you want to convey:

* "Tú" is the informal form, used with friends, family, children, and generally anyone you're close with.

* "Ti" is the formal form, used with strangers, people in positions of authority, and anyone you want to show respect to.

Here's an example:

* "Hola, ¿cómo estás tú?" (Hello, how are you?) - This is informal, suitable for a friend.

* "Hola, ¿cómo está usted?" (Hello, how are you?) - This is formal, suitable for a teacher or a stranger.

It's important to use the correct form to avoid misunderstandings and show respect.