Learn French: Garde ce secret pour toi. - Keep this secret to yourself.

This is a command (the imperative mood) directed at one person you know well. For regular verbs ending in -er like garder, you drop the final -s from the tu form when giving a command. So, tu gardes becomes just Garde.

Why do we use toi here instead of te or tu?

Because it comes right after a preposition, which in this case is pour (for/to). In French, you must use a stress pronoun after prepositions. The stress pronoun for tu is toi.

Why does it say pour toi instead of translating 'to yourself'?

Languages express ideas differently! In French, to say you are keeping something 'to yourself,' you literally say you are keeping it 'for you' (pour toi). You don't need a special word for 'yourself' in this phrase.

Why is it ce secret instead of le secret?

Ce is a word meaning 'this' or 'that' for masculine singular nouns. Since secret is masculine, we use ce to point out 'this secret' specifically. If you used le, it would just mean 'the secret'.