Learn Finnish: Minä piilotin sormuksen hirviöiltä. - I hid the ring from the monsters.

The base word is hirviö (monster). To say "from the monsters", we first add the plural marker -i- and then the ablative case ending -ltä. We use -ltä instead of -lta because of vowel harmony, since the word contains the front vowel ö.

Why do we use the ablative case (-lta/-ltä) for the monsters?

In English, we hide things "from" people, and Finnish uses the ablative case (which often translates as "from a surface" or just "from") for this exact same idea. When you hide something from someone using the verb piilottaa, the person or thing you are hiding it from takes the ablative ending.

The dictionary form of "to hide" is piilottaa. Why is there only one "t" in piilotin?

This is due to consonant gradation. When adding the personal ending -n (for "I"), the syllable is closed off. This triggers a change in the middle of the word from the strong grade tt to the weak grade t.

Why does the word sormus (ring) become sormuksen here?

Two things are happening! First, the ring is the object of the sentence, so it takes the -n ending to show that the action is complete. Second, nouns ending in -us usually change to an -ukse- stem before you can add any endings, which gives us the stem sormukse- plus the ending -n.