Publiken är högljudd under konserten.

Because publiken is a collective noun. It refers to a group of people, but grammatically it is treated as singular in Swedish, just like the audience is usually singular in English.

• publik = audience
• publiken = the audience

So in this sentence, Swedish treats the whole audience as one group.

The ending -en is the definite article attached to the noun.

In Swedish, instead of putting a separate word like the before the noun, you often add the definite ending to the noun itself.

• publik = audience
• publiken = the audience

This is very common in Swedish.

Because Swedish usually marks definiteness with an ending on the noun.

So:

• publiken = the audience
• konserten = the concert

English uses a separate word (the), but Swedish often uses a suffix instead.

Är is the present tense of vara, which means to be.

So:

• jag är = I am
• du är = you are
• publiken är = the audience is

Unlike English, Swedish does not change the present tense form depending on the subject. It is always är.

Because adjectives in Swedish must agree with the noun they describe.

Here, högljudd is describing publiken, which is:

• singular
• common gender (en-word)

So the correct form is högljudd.

Compare:

• en publik är högljudd = an audience is loud
• ett barn är högljutt = a child is loud
• människorna är högljudda = the people are loud

Since publiken is singular and an en-word, högljudd is the right form.

Högljudd means loud or noisy.

It is often used for people or groups making a lot of sound. Depending on context, it can sound:

• neutral: simply loud
• slightly negative: noisy, too loud

In this sentence, it suggests that the audience is making a noticeable amount of noise during the concert.

Here under means during.

So:

• under konserten = during the concert

Swedish under can also mean under/below, depending on context, but in this sentence it clearly means during.

It actually is under konserten — with -en on konsert.

• konsert = concert
• konserten = the concert

So under konserten means during the concert.

The noun is in its definite form because the sentence refers to a specific concert.

Normally, no. In standard Swedish, publiken is grammatically singular, so the adjective should also be singular:

• Publiken är högljudd.

Even though an audience contains many people, Swedish usually agrees with the grammatical form of the noun, not the real-life number of people.

If you want a plural adjective, you would usually need a plural noun, for example:

• Människorna i publiken är högljudda.
  = The people in the audience are loud.

This is the normal Swedish main-clause word order:

subject + verb + complement + adverbial

So here:

• Publiken = subject
• är = verb
• högljudd = complement
• under konserten = time phrase / adverbial

Swedish is a V2 language, which means the finite verb usually comes in the second position in main clauses. Since the subject comes first here, är comes second.

Yes:

• Under konserten är publiken högljudd.

That is also correct. When a phrase like under konserten comes first, Swedish still keeps the verb in second position, so är must come before publiken.

This is a very important Swedish word-order pattern.

A rough guide is:

• hög sounds a bit like hug with rounded lips
• ljud contains the Swedish lj sound, which can be tricky for English speakers
• högljudd is roughly HURG-lyood or HURG-yood, but that is only approximate

A more accurate pronunciation in IPA is approximately:

• [ˈhøːjˌjʉdː] or similar, depending on accent

The hardest parts for English speakers are usually:

• ö
• the lj sound
• the long consonant at the end

Yes. The basic noun is en konsert.

So:

• en konsert = a concert
• konserten = the concert
• konserter = concerts
• konserterna = the concerts

This is why the definite singular ending is -en.

It most naturally describes a situation in that context: the audience is loud during the concert.

Because of under konserten, it sounds like a statement about what is happening in that period of time, not a permanent characteristic of the audience.

So the sentence feels situational rather than general.