De opptrer i kveld.

At the beginning of a sentence, De could be either:

• de = they (normally lowercase, but capitalized at the start of a sentence)
• De = formal you (an old-fashioned or very formal written form)

In modern everyday Norwegian, de meaning they is much more common. Since the meaning has already been shown to you, that tells you which one is intended here.

Because it is the first word in the sentence.

If the word is the pronoun de = they, it is normally written with a lowercase d, but sentence-initial capitalization makes it De.

There is one extra complication: capital De can also be a formal way of writing you. So at the start of a sentence, De is visually ambiguous, and you need context to know which meaning is meant.

opptrer is the present tense form of the verb å opptre.

So:

• å opptre = to perform / to appear / sometimes to behave
• opptrer = perform(s), is performing, appears, behaves

In this sentence, with i kveld, it most naturally means perform.

The verb å opptre can have a few related meanings, depending on context:

• to perform: Bandet opptrer i kveld = the band is performing tonight
• to appear: someone appears in a show or event
• to behave / act: Han opptrer rolig = he behaves calmly

In De opptrer i kveld, the time expression i kveld strongly suggests the perform meaning.

Because Norwegian does not use a separate auxiliary here the way English does in they are performing.

Norwegian often uses the simple present where English uses either:

• they perform
• they are performing

So De opptrer can naturally cover both ideas, depending on context.

Because Norwegian often uses the present tense for scheduled or planned future events.

So De opptrer i kveld can mean:

• They perform tonight
• They are performing tonight
• They will perform tonight

This is very normal in Norwegian.

If you want to make the future idea more explicit, you can also say:

• De skal opptre i kveld = they are going to perform tonight / they will perform tonight

Literally:

• i = in
• kveld = evening

But together i kveld is a fixed time expression meaning:

• tonight
• this evening

Norwegian often uses these simple time phrases without an article.

Because the two expressions mean slightly different things.

• i kveld = tonight / this evening
  A specific evening, usually the current one.

• på kvelden = in the evening
  More general, or describing something that happens during the evening as a time period.

So for They are performing tonight, i kveld is the natural choice.

Yes. This is standard Norwegian word order:

• De = subject
• opptrer = verb
• i kveld = time expression

So the pattern is:

subject + verb + time expression

You can also move the time expression to the front:

• I kveld opptrer de.

When that happens, Norwegian keeps the verb in the second position. This is a very important pattern in Norwegian word order.

A rough guide:

• De: often like di when it means they, though pronunciation varies by dialect
• opptrer: roughly OP-trer
• i kveld: roughly ee KVELD

A few useful pronunciation points:

• The double pp in opptrer makes the vowel before it short.
• The kv in kveld is pronounced; do not drop the k.
• The r sound varies a lot across Norwegian dialects.

So a rough overall approximation could be:

di OP-trer i KVELD

But real pronunciation depends on dialect, and English spelling can only give a rough idea.