Learn Finnish: Tämä suklaa on todella maidotonta. - This chocolate is really dairy-free.

The Finnish word for milk is maito. To add the suffix -ton (meaning 'without' or '-less'), we use the weak stem of the word. The consonant t weakens to d, making the base maidoton (dairy-free). Finally, the partitive ending -ta is added to the end.

Why is the adjective in the partitive case (maidotonta) instead of the basic form (maidoton)?

This happens because suklaa (chocolate) is a mass noun, meaning it is an uncountable substance. In Finnish, when describing a mass noun with the verb olla (to be), the adjective that follows must always be in the partitive case.

Is there a special rule for adding the partitive ending to -ton adjectives?

It is very straightforward! For adjectives ending in the suffix -ton or -tön, you simply add the partitive ending -ta or -tä directly to the basic form of the word. That is why maidoton becomes maidotonta.

Could I use tosi instead of todella here?

Yes! Both todella and tosi mean 'really' or 'very'. Todella is the standard written form, while tosi is extremely common in everyday spoken Finnish. Both are perfectly fine to use in this sentence.