Starting in verse 29, John introduces the idea of being born into God's family. The previous verses show that believers continue to sin; this part shows that believers also have the new nature, which cannot sin. It continues to show how believers can recognize each other.
Here the word "everything" is a generalization. Alternate translation: "because his anointing teaches you everything that you need to know" (See: [[rc://en/ta/man/translate/figs-hyperbole]])
To remain in someone means to continue to have fellowship with him. See how you translated "remains in God" in [1 John 2:5-6](./04.md). Alternate translation: "continue to have fellowship with him" or "stay joined with him" (See: [[rc://en/ta/man/translate/figs-metaphor]])