Learn Finnish: Sika ei ole aasia itsepäisempi. - The pig is not more stubborn than the donkey.

This sentence uses the partitive comparative structure you are currently learning. Instead of using kuin (than) and the basic nominative case (kuin aasi), you can simply put the thing being compared against into the partitive case (aasia). Both ways mean exactly the same thing.

Why does the partitive word aasia come before the adjective itsepäisempi?

When using this partitive comparative structure without kuin, the standard of comparison (aasia) almost always comes immediately before the comparative adjective (itsepäisempi). If you used kuin, the order would be reversed: itsepäisempi kuin aasi.

How is the comparative word itsepäisempi formed from the basic adjective itsepäinen (stubborn)?

For adjectives ending in -nen, you first find the word stem by dropping the -nen and replacing it with -se-. This gives you the stem itsepäise-. Then you just attach the standard comparative ending -mpi, giving you itsepäisempi.

If the sentence was positive ('The pig is more stubborn than the donkey'), would the partitive structure still be exactly the same?