Learn Finnish: Onko tämä koe todella vaikea? - Is this exam really difficult?

Koe belongs to the group of Finnish words ending in -e. To form the stem, you double the final vowel and apply consonant gradation. The stem is koke-, which gives forms like the genitive kokeen, the partitive koetta, and the elative kokeesta (which you would use if you paired this word with an emotion verb).

If I want to use an emotion verb like stressata (to stress) with this subject, how would tämä koe change?

Because emotion verbs often govern the elative case, you would need to put both the pronoun and the noun into the elative. Tämä koe would become tästä kokeesta (e.g., Stressaan tästä kokeesta). In our current sentence, they stay in the basic nominative case because they are simply the subject of the verb olla (to be).

Could I use tosi instead of todella in this sentence?

Yes, in spoken language, tosi vaikea is perfectly natural and very common. However, todella is formally an adverb (literally meaning "with truth" in the adessive case) and is the standard, preferred choice in written or slightly more formal Finnish.

Why is vaikea in the basic nominative case rather than the partitive vaikeaa?