Short answer: "That" is used to introduce essential clauses or information, while the pronoun "which" is used to add non-essential or non-defining clauses. Also, these non-essential clauses are set off with commas.
Explanation:
First of all, the words "that" and "which" are both relative pronouns; this means, both introduce clauses that describe a noun previously mentioned. However, the pronoun "that" should be used only to introduce essential or defining clauses (information that cannot be omitted); for example "She is the woman that I love". On the other hand, the word "which" introduces information that is not essential or non-defining clauses; for example "The house, which is located near the sea, is stunning". Moreover, clauses introduced by which are set off in commas to show they are not essential.
Percy Jackson is one of my favorite series! I highly recommend reading the rest of the series.
1. Percy is taken to Camp Half-Blood and defeats his first monster
2. Percy finds out his father is Poseidon
3. Percy is given a prophecy
4. Percy goes on the quest with Grover and Annabeth
5. Percy finds his mom in the Underworld
6. Hades tells Percy about his stolen Helm
Climax: Percy finds out who the Lightning Theif is
