Learn Finnish: Ostin kauniin matkamuiston Virosta. - I bought a beautiful souvenir from Estonia.

The verb is ostaa (to buy). To form the past tense, you start with the basic stem osta-. For two-syllable Type 1 verbs where the first vowel is o or u, the final a of the stem changes to i in the past tense. So, osta- becomes osti-, and with the -n ending for 'I', it becomes ostin.

Why do kauniin and matkamuiston both end in -n?

They make up the object of the sentence (the thing you bought). Because you bought one complete souvenir, the object takes the -n ending (the accusative case). The adjective kaunis (beautiful) must match the noun it describes, so it also takes the -n ending.

How does the basic adjective kaunis become kauniin?

Words ending in -is, like kaunis, have a special stem. When you add endings to them, the -is changes to a double -ii-. So the stem of kaunis is kaunii-, and when you add the -n object ending, you get kauniin.

Why use Virosta to say 'from Estonia' rather than another 'from' ending?