Learn Portuguese (Portugal): Ela chegou cedo, o que me surpreendeu.

When 'which' refers back to a complete idea or action rather than a specific noun, you must use o que. Here, it refers to the entire previous phrase: the fact that she arrived early. If you used just que, it would need a noun right before it to refer to.

Why is the pronoun me placed before the verb (me surpreendeu) instead of after (surpreendeu-me)?

In European Portuguese, object pronouns like me usually attach to the end of the verb. However, certain words act as magnets and pull the pronoun before the verb. The word que is a very strong magnet, so it forces the order to be que me surpreendeu.

Does the word cedo change if the subject is plural?

No. Cedo is an adverb meaning 'early,' which describes when the action happened. Adverbs do not have gender or plural forms, so it always stays exactly the same, even if the sentence was Eles chegaram cedo.