Learn Swedish: Är det Anna som regisserar serien? - Is it Anna who directs the series?

The word vem is only used as a question word, like when asking "Who is that?". When "who" or "that" connects a description back to a noun, acting as a relative pronoun, you must use som. Here, som links Anna to the action she is doing.

Can I leave out som here, since we sometimes skip "who" or "that" in English?

No, you must keep it in this sentence. In Swedish, you can only drop som if it is the object of the relative clause. Because som is the subject here (it represents the person actually doing the directing), it cannot be removed.

Does the word som change if the subject is plural?

No, som is completely inflexible. It means "who", "which", or "that" for any noun, regardless of whether the word it refers back to is an en-word, an ett-word, singular, or plural.

Why does the sentence start with Är det?

This is how you form a yes/no question in Swedish. You take a normal statement like Det är Anna (It is Anna) and swap the subject and the verb to put the verb first, creating Är det Anna? (Is it Anna?).

Why does the word for "the series" end in just -n instead of -en?