Learn Finnish: Sinun kannattaa suostua uuteen liittoumaan. - You should consent to a new alliance.

The verb suostua (to consent, to agree) requires the illative case (the "into" case) for the thing being agreed to. While English uses the preposition "to", Finnish treats consenting as stepping into an agreement, plan, or situation.

Why is the subject "you" in the genitive case (sinun) instead of the nominative (sinä)?

The verb kannattaa (meaning "should" or "to be worthwhile") is a necessity verb. In Finnish, necessity verbs require the subject to be in the genitive case. The sentence structure translates literally to something closer to "It is worthwhile for you to consent."

Uusi means "new". Why does it change so drastically to uuteen in the illative case?

Old Finnish words ending in -si, such as uusi, vesi, or käsi, have a consonant stem ending in -te-. To form the illative, you take this stem (uute-) and add the illative ending -en, resulting in uuteen.

I've previously learned the word "liitto" for "alliance" or "union". How is "liittouma" different?

While liitto is a broad term for a union, league, or federation (like a trade union or the EU), liittouma typically refers to a coalition, bloc, or a specific strategic alliance (often political or military). It emphasizes the group having allied together for a specific, sometimes temporary, purpose.