When do I use these words?
How do I know when to use vous and tu and words like that?
If you think about it as strictly "formal" (Vous) and "informal" (Tu), this is missing the contextual subtlety of this of this French language idiosyncrasy. It's not just knowing what Vous is, Il s'agit de knowing how and when to use it.
Par example - when referring to a group, when using an imperative command, when in a job interview... etc etc. Learners think that Vous conjugation is unnecessary because you can just use "Tu" most of the time but to speak French well you have to understand when it is necessary to use Vous and so it's important to know how to use it.