Learn German: Jede Zeile, welche du korrigierst, ist perfekt. - Every line which you are correcting is perfect.

In German, welcher, welche, and welches can be used as relative pronouns instead of der, die, and das. They mean exactly the same thing, but they belong to a more formal, written register. Using welche here makes the sentence sound a bit more elevated or official than die would.

How do I know to use the exact form welche in this sentence?

A relative pronoun takes its gender from the noun it refers to (the feminine noun Zeile). Its case, however, comes from its grammatical job inside the relative clause. Here, it is the direct object of korrigierst (you are correcting the line), making it accusative. The feminine accusative form of this pronoun is welche.

Are there times when I have to use welche instead of die?

You never strictly have to use it, but native speakers often choose a form of welcher to avoid awkward repetition. For example, if a sentence naturally produces a double "die" (like die, die wir sehen - "the one that we see"), they might swap the second one to welche (die, welche wir sehen) just to make it flow better. In our current sentence, Zeile, die du... doesn't repeat any words, so welche is purely a stylistic choice for formality.