På pikniken drikker vi saft og spiser pærer, blåbær og bringebær.

The ending -en is the definite ending for many masculine nouns in Bokmål. So:

• en piknik = a picnic
• pikniken = the picnic

In this sentence, it refers to a specific picnic, not just any picnic.

This is because of the Norwegian V2 rule: in a main clause, the finite verb usually comes in the second position. Here, På pikniken is placed first, so the verb drikker must come next, and the subject vi comes after it.

Compare:

• Vi drikker saft.
• På pikniken drikker vi saft.

Both are correct, but the second one starts by emphasizing the setting.

Because the same subject, vi, applies to both verbs. Norwegian works like English here: when two verbs are joined by og, you usually do not repeat the subject.

So drikker vi saft og spiser pærer means we drink juice and eat pears, with we understood for both actions.

Because saft is being used as a mass noun here, like juice, water, or tea in English. Norwegian often leaves out the article with drinks and substances when speaking generally.

For example:

• drikker kaffe
• drikker te
• drikker saft

Because Norwegian usually does not use an article with indefinite plural nouns in a general statement. So:

• spiser pærer = eat pears
• spiser blåbær = eat blueberries
• spiser bringebær = eat raspberries

If you wanted to stress some, you could add a word like noen, but it is not needed here.

This is just how those nouns form their plurals.

• en pære → pærer
• et blåbær → blåbær
• et bringebær → bringebær

Some Norwegian nouns, especially many neuter nouns, have the same form in singular indefinite and plural indefinite.

This is normal list punctuation in Norwegian. Items in a list are separated by commas, but there is usually no comma before og.

So pærer, blåbær og bringebær is the standard way to write the list. This is similar to English without the Oxford comma.

Yes, that is also grammatical. The difference is mainly emphasis.

• På pikniken drikker vi ... puts the picnic first.
• Vi drikker ... på pikniken puts the actions first.

Both are natural, but the original sentence highlights the setting right away.