Learn Finnish: Suostutko sinä todella tuollaiseen petokseen? - Do you really consent to that kind of betrayal?

This is the core of the current lesson! The verb suostua (to consent, to agree to) always requires the illative case for the thing being agreed to. Because of this rule, both the noun petos (betrayal) and its modifier tuollainen (that kind of) take the illative ending.

How is the illative form petokseen formed from the basic word petos?

Nouns ending in -os or -ös (which are often derived from verbs, like pettää meaning "to betray") change their ending to -okse- or -ökse- to form their stem. For the illative case, you lengthen the final stem vowel and add -n, resulting in petos -> petokse- -> petokseen.

Tuollainen becomes tuollaiseen. How does that stem change work again?

Words ending in -nen form their stem by replacing -nen with -se-, giving us tuollaise-. Just like with petokseen, the illative is formed by lengthening the final stem vowel and adding -n, which makes tuollaise- + -en = tuollaiseen.

Since suostutko already means "do you consent," why is the pronoun sinä included here?