Learn Finnish: Onko lehmä hevosta painavampi? - Is the cow heavier than the horse?

This sentence uses the partitive comparative structure. Instead of using the word kuin (than) followed by the basic nominative form (kuin hevonen), you can drop kuin and put the thing being compared to in the partitive case (hevosta). Both mean "than the horse".

Why does hevosta come before the adjective painavampi?

In this partitive comparative structure, the word in the partitive case (hevosta) must be placed immediately before the comparative adjective (painavampi). If you used the kuin structure, the order would be the other way around: painavampi kuin hevonen.

Could I use kuin in this sentence instead?

Yes, absolutely! You could say Onko lehmä painavampi kuin hevonen? and it would mean exactly the same thing. The partitive structure (hevosta painavampi) is just a slightly shorter, very common alternative you will hear native speakers use.

How do we get hevosta from the basic word hevonen?

This is a good reminder for words ending in -nen. To form the partitive, you replace the -nen ending with -sta or -stä. Therefore, hevonen becomes hevosta.