Learn Finnish: Auringon muoto on ympyrä. - The shape of the sun is a circle.

It is in the genitive case because it means 'of the sun' or 'the sun's'. Adding the -n ending closes the final syllable, which triggers consonant gradation. The strong nk combination softens to ng, changing aurinko to auringon.

Why is the order auringon muoto instead of muoto auringon?

In Finnish, the genitive modifier (the word with the -n ending showing possession or connection) almost always comes directly before the noun it describes. Even though English says 'the shape of the sun', Finnish structures it like 'the sun's shape'.

How would this sentence change if I just wanted to say 'The sun is round'?

You would drop the word muoto (shape) and use the adjective pyöreä (round) instead of the noun ympyrä (circle). The sentence would simply be Aurinko on pyöreä.

Is there a common alternative way to say 'The sun is circle-shaped'?

Yes, you can use the adjective suffix -muotoinen (shaped). You put the base shape in the genitive case before it, so 'circle-shaped' becomes ympyrän muotoinen. A very natural way to say this is Aurinko on ympyrän muotoinen.