Learn Hardcore Norwegian: Et ekstra rom gjør huset større.

gjør is the present tense of the verb gjøre (“to make” or “to do”). Here it translates as “makes” – so the sentence means “An extra room makes the house bigger.”

Why is it et ekstra rom and not det ekstra rommet?

Because you’re talking about any extra room (indefinite), not a specific one. rom is a neuter noun, so the indefinite article is et. Use det ekstra rommet only when you mean “that extra room” (definite).

How do you form the comparative større from stor?

stor means “big.” To make the comparative, you add -re and change o to ø, giving større (“bigger”). Many adjectives in Norwegian form comparatives by adding -ere, but stor is irregular.

Could you say mer stor instead of større?

No. Some adjectives use mer + adjective (like mer interessant), but stor has its own comparative større. mer stor would be ungrammatical.