Regnvannstønnen er full.

It is a compound noun:

• regn = rain
• vann = water
• tønne = barrel / cask

So regnvannstønne literally means rainwater barrel.

Norwegian makes compounds very freely, much like English. Instead of writing rain water barrel as separate words, Norwegian normally joins them into one word: regnvannstønne.

The ending -en marks the noun as definite singular, meaning the rainwater barrel.

So:

• en regnvannstønne = a rainwater barrel
• regnvannstønnen = the rainwater barrel

In Norwegian, definiteness is often shown by adding an ending to the noun rather than using a separate word like the.

Because Norwegian usually builds the into the noun itself.

So English:

• the barrel

becomes Norwegian:

• tønnen

And in this sentence:

• Regnvannstønnen = the rainwater barrel

This is one of the biggest differences from English grammar.

Er is the present tense of å være (to be).

So:

• jeg er = I am
• du er = you are
• den er = it is

In Regnvannstønnen er full, er simply means is.

Because full is a predicate adjective, and it agrees with the gender/number of the noun.

Here, regnvannstønnen is a singular common gender noun, so the adjective is:

• full

Compare:

• Regnvannstønnen er full. = The rainwater barrel is full.
• Huset er fullt. = The house is full. (hus is neuter)
• Tønnene er fulle. = The barrels are full. (plural)

So full is correct because the subject is singular common gender.

Often, yes.

The noun tønne is feminine, and in Bokmål many feminine nouns can be treated in two ways:

• feminine definite singular: tønna
• common gender definite singular: tønnen

So both of these can be possible in Bokmål:

• Regnvannstønna er full.
• Regnvannstønnen er full.

Which one you use depends on the variety of Bokmål and your personal style. -en forms are often more neutral/safe for learners, but -a forms are also very common.

Because full is describing the subject through the verb to be.

This is the same basic pattern as in English:

• The rainwater barrel is full.

Norwegian uses the same kind of structure:

• Regnvannstønnen er full.

So the pattern is:

• subject + er + adjective

It can describe both, but the meaning depends on context.

For objects or containers, full usually means filled up:

• Glasset er fullt. = The glass is full.

For people, full often means drunk in everyday language:

• Han er full. = He is drunk.

In Regnvannstønnen er full, the context clearly shows it means the barrel is filled up.

A careful approximate pronunciation is:

• Regnvannstønnen er full
• roughly: RAIN-vahn-stUHN-nen ehr fool

A few useful notes:

• regnvann is a compound, and the main stress is toward the beginning.
• ø in tønnen is a vowel English does not really have; it is somewhat like the vowel in French deux or German schön.
• u in full is also not the same as English oo in food, though English speakers often use that as a rough approximation at first.

You do not need perfect pronunciation right away; the important thing is recognizing the compound and the endings.

The pattern is:

• [definite noun] + [present tense of “to be”] + [adjective]

So:

• Regnvannstønnen = the rainwater barrel
• er = is
• full = full

This is a very common and useful Norwegian sentence type, like:

• Boka er interessant. = The book is interesting.
• Døra er åpen. = The door is open.
• Kaffen er varm. = The coffee is warm.