Korøvelsen begynner klokken sju.

Because Norwegian often puts the definite article at the end of the noun.
So:

• en korøvelse = a choir rehearsal
• korøvelsen = the choir rehearsal

The ending -en here marks singular definite.

Yes. Norwegian very often makes one compound noun where English would use two words.
Korøvelsen is built from:

• kor = choir
• øvelse = practice / rehearsal

So Norwegian writes it as one word, not as separate words.

Begynner is the present tense of å begynne = to begin.

Examples:

• Jeg begynner = I begin / I am beginning
• Hun begynner = She begins / she is beginning
• Korøvelsen begynner = The choir rehearsal begins

Norwegian uses the same present-tense form for all subjects, so you do not change the verb the way English changes begin to begins.

Because Norwegian usually does not need helper verbs here the way English sometimes does.

English might say:

• The choir rehearsal begins at seven
• The choir rehearsal is beginning at seven
• The choir rehearsal does begin at seven for emphasis

Norwegian simply uses begynner in a normal sentence like this. The extra helper verbs are not needed.

Literally, klokken means the clock, but in time expressions it is used the way English uses at with clock times.

So:

• klokken sju = at seven o’clock

It is a very common pattern in Norwegian when giving an exact time.

Because Norwegian normally does not use a separate preposition before an exact clock time in this structure.

You simply say:

• begynner klokken sju

not something like begynner på klokken sju.

This is just the standard Norwegian way to express at seven o’clock.

Yes, in Bokmål both sju and syv are used for seven.

So these are both possible:

• klokken sju
• klokken syv

Sju is also the normal form in Nynorsk.

Yes. Norwegian main clauses follow the verb-second rule, so if you move the time expression to the front, the verb still stays in second position.

You can say:

• Korøvelsen begynner klokken sju.
• Klokken sju begynner korøvelsen.

Both are correct. The second version puts more focus on the time.

The letter ø is a vowel that English does not really have.

A rough guide:

• it is similar to the vowel in French peu
• or German ö

Your lips are rounded, but the tongue is placed more forward than for English o.

In korøvelsen, the ø sound appears in the second part of the word. It is worth practicing separately, because ø is different from both o and u in Norwegian.

Yes, very often you can.

• Korøvelsen begynner klokken sju.
• Korøvelsen starter klokken sju.

Both are natural. Begynner is a very basic and common verb meaning begins, while starter often feels like starts. In this sentence, either works well.