Une soirée mouvementée (Passé Composé with être)

Question

The 1st sentence in this fill-in-the-blank quiz was " Laissez-moi vous raconter ce qu'il nous........" Translated as, "Let me tell you what happened to us! My response was conjugated to Nous, and so, "sommes arrivés" was incorrect.  Should I have conjugated the answer with moi, the subject of the verb? The correct answer was "est arrivé" . Could you please explain why IL became the subject?    Thanks.

Accepted Answer

Arriver doesn't just mean "to arrive", it also has some figurative meanings. "Il arrive" using an impersonal "il" means "it happens", and "il nous arrive" would mean "it happens to us". In this case "nous" is the object, not the subject of the verb. Therefore "ce qu'il nous est arrivé" means "what happened to us".