<span>Oedipus main motivation was to end the plague that ravished his land, a seer tells him to find the previous king's killer. Oedipus vows to do so</span>
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.
