Learn Swedish: Både cellon och fiolen är dyra instrument. - Both the cello and the violin are expensive instruments.

Just like in English, Swedish often uses the definite singular form to talk about an entire class of instruments, species, or inventions. You could also use the indefinite plural here to express the same general idea.

Why is the definite form of cello just cellon instead of taking the usual -en suffix?

Cello is an en-word, but because it is a loanword that already ends in a vowel, adding the standard -en suffix would create an awkward double vowel. Instead, it just adds an -n to become cellon.

Why doesn't the word instrument have a plural ending here?

Instrument is an ett-word that ends in a consonant (ett instrument). In Swedish, these types of words do not change their spelling in the indefinite plural. We know it is plural here because it refers to two things and is paired with the plural adjective dyra.

Since instrument is an ett-word, why is the adjective dyra instead of dyrt?