Learn Finnish: Me syötiin eväät ulkona. - We ate our snacks outside.

In colloquial Finnish (puhekieli), the passive is almost always used for the first-person plural (me). Instead of the standard me söimme, spoken Finnish uses me syötiin. Because the action happened in the past, it uses the past passive form.

Why is the object eväät and not the partitive eväitä?

Using the plural nominative (eväät) makes this a total object, meaning the action was fully completed. It implies we ate all of our snacks. If you used the partitive (eväitä), it would mean we just ate some of the snacks, or that we were still in the middle of eating them.

What is the singular dictionary form of eväät?

The singular form is eväs (a snack or packed lunch). Words ending in -s often change their stem to end in a double vowel, so the stem becomes evää-. To make it plural, you just add the plural marker -t.

Why does the translation say 'our snacks' when there is no word for 'our' in the Finnish sentence?

In Finnish, you don't always need to explicitly state ownership if it's obvious from the context. Because the subject is me (we), it is naturally understood that the snacks belong to the speakers. In spoken Finnish you could also add the genitive pronoun and say meidän eväät, but just eväät is perfectly natural here.