Learn Swedish: Skådespelaren glömde repliken, vilket var uselt. - The actor forgot the line, which was awful.

In Swedish, when "which" refers back to an entire situation or clause (the fact that the actor forgot the line) rather than a specific noun, you use vilket or något som. If you used som here, it would refer only to the noun immediately before it (repliken), which would change the meaning to say that the script line itself was awfully written, rather than the act of forgetting it being awful.

Could I use något som instead of vilket in this sentence?

Yes! You can absolutely say , något som var uselt (literally: ", something that was awful"). Both vilket and något som are used to refer back to a whole clause. Vilket can sound slightly more formal or better suited for writing, while något som is very common in everyday speech.

Why does uselt end in a -t when both skådespelaren and repliken are en-words?

The adjective uselt (awful/lousy) is describing the entire situation of forgetting the line, which is represented by the relative pronoun vilket. In Swedish, abstract concepts and entire clauses are treated as neuter (ett words), so adjectives describing them must take the neuter -t ending.

Does repliken just mean "the reply"?