Learn Finnish: Aito jalokivi erottuu heti kopiosta. - A genuine gem stands out from a copy immediately.

This is due to verb rection. Verbs related to separating, distinguishing, or differing, such as erottua (to stand out) and erottaa (to separate), require the entity you are separating from to be in the elative case.

The lesson mentions the verb erottaa. Why does this sentence use erottuu instead?

Erottaa is transitive, meaning someone actively separates or distinguishes an object from something else. Erottua is its intransitive counterpart, meaning something distinguishes itself or stands out. Since the gem is doing the standing out on its own, we use erottua. Both verbs share the same elative rection.

Is jalokivi a compound word?

Yes, it is made of jalo (noble, precious, fine) and kivi (stone). Together they literally mean precious stone, or gem.

Can I change the word order and put heti at the end of the sentence?

Yes, Aito jalokivi erottuu kopiosta heti is grammatically correct. However, placing the adverb heti right after the verb makes the sentence flow very naturally, whereas putting it at the end places a slightly stronger emphasis on the immediacy itself.