Learn Finnish: Me maistoimme kylmää ruokaa.

The base verb is maistaa (to taste). To form the past tense, you take the stem maista- and add the past tense marker -i-. For verbs ending in -aa, that final a changes into an o before the -i-. Finally, you add the personal ending -mme (we), giving us maistoimme.

Why is the object in the partitive case (kylmää ruokaa) instead of the accusative?

The verb maistaa is a partitive verb. This means that whatever you are tasting must always take the partitive case, regardless of whether you tasted the whole thing or just a small bite.

How do we form the partitive for kylmä and ruoka here?

Both words end in a single short vowel. To make them partitive, you just add another matching vowel to the end. kylmä becomes kylmää and ruoka becomes ruokaa. The adjective must always match the case of the noun it describes.

Is it necessary to include the word Me at the start of the sentence?

No, it is entirely optional. Because the verb ending -mme already means we, you could just say Maistoimme kylmää ruokaa. Including me just adds a bit of emphasis to who did the tasting.