Informal means without formality or ceremony. Casual.
Informal means not formal. This means you use casual, contracted, and colloquial terms in your writing/speaking. You can also say dressing up in casual clothes is informal.
Formal.
Tu means you (singular and informal)
Informal
It means "You can" using the informal version of you.
Formal means that you need to be dressed up for the interview. Informal means that you can come in casual clothing, I wouldn't suggest jeans.
Social Control
Habla means, you speak. (informal)
Note.
offhand
Gobsmacked - is an informal word meaning 'speechless'.
It means "you do" (plural, informal version).
Because phono means informal which means a suitable speech and/or writing.