Learn Russian: Что-то случилось ночью.

Что usually means "what" (a question word) or "that" (a conjunction). Adding the suffix -то turns it into the indefinite pronoun "something". The speaker knows an event occurred but doesn't know exactly what it was.

I am learning about these pronouns. Could I say что-нибудь случилось ночью instead?

It would sound strange here. We use words ending in -то when talking about a specific but unknown thing that actually happened (a fact). We use words ending in -нибудь (meaning "anything") for hypothetical situations, questions, or future events. Since this happened in the past and is a real fact, что-то is the correct choice.

Why does the verb случилось end in -ось?

Two things are happening here! First, the pronoun что-то (just like что) is grammatically neuter, so it takes the neuter past tense ending -о. Second, случиться (to happen) is a reflexive verb. Because the neuter ending -о is a vowel, the reflexive suffix -ся shrinks to -сь, giving us случилось.

Why is the word for "night" (ночь) written as ночью here?