Learn Swedish: Det är oljudet som stör oss när vi sover. - It is the noise that disturbs us when we sleep.

The prefix o- often works like "un-" or "non-" in English to create a negative or opposite meaning. Here, it attaches to ljud (sound) to create oljud (bad sound, racket, or noise). The -et at the end makes it the definite form, so oljudet means "the noise".

Does oljud mean exactly the same thing as the English word "noise"?

It translates to "noise", but oljud carries a strong sense of being an unpleasant or annoying sound. For general loud background noise, like a busy road or machinery, Swedish often uses the word buller.

Why is som used for "that" in this sentence instead of att?

The word som is a relative pronoun used to link back to a specific noun, which in this case is oljudet. You use som when "that" acts as the subject doing the action (disturbing). The word att is used to introduce a completely new clause, like in "I think that..." (Jag tycker att...).

What is the purpose of starting with Det är... instead of just saying Oljudet stör oss?

Starting with Det är (It is) and following up with som creates a structure that emphasizes a specific part of the sentence. It highlights that the noise, specifically, is what is disturbing you.