Learn Finnish: Kivitalo on todella kylmä talvella. - A stone house is really cold in the winter.

In Finnish compound words, when the first word describes the material or the type of the second word, it usually stays in the basic nominative case. Since the house is made of stone, we use kivi + talo. If the first word expressed a purpose or an intended user (like a room for children), it would take the genitive case, as in lastenhuone.

Could I just say kivinen talo using an adjective instead of a compound noun?

Yes, kivinen talo is grammatically correct and means a house made of stone. However, Finnish strongly prefers compound nouns for established categories of things. Kivitalo (stone house) and puutalo (wooden house) are standard, everyday categories of buildings, so the compound form is much more natural to a native speaker.

Why is talvella in the adessive case (-lla) to mean 'in the winter'?

Seasons in Finnish use the adessive case to express when something happens during that general time. Just like kesällä (in the summer) or syksyllä (in the autumn), talvella is the standard way to say 'in the winter.' At B2, it is good to remember that while days and exact dates often take the essive case (-na), seasons and parts of the day typically take the adessive (-lla/-llä).

Can I use tosi instead of todella in this sentence?