Dansepartneren ser på speilveggen.

Because -en is the usual definite singular ending for many Norwegian nouns of common gender.

• en dansepartner = a dance partner
• dansepartneren = the dance partner

Unlike English, Norwegian often puts the onto the end of the noun instead of using a separate word.

A useful extra point: if you add an adjective, Norwegian often uses double definiteness:

• den flinke dansepartneren = the skilled dance partner

Ser is the present tense of å se = to see / to look.

So:

• å se = infinitive
• ser = present tense

Norwegian verbs do not change for person the way English verbs sometimes do. So you get:

• jeg ser
• du ser
• han/hun ser
• vi ser

All use the same present-tense form: ser.

Because å se på often means to look at or to watch, while å se by itself more often means to see.

So there is an important difference:

• ser = sees
• ser på = looks at / is watching

In this sentence, the idea is that the dance partner is directing their gaze toward the mirror wall, so ser på is the natural choice.

Compare:

• Dansepartneren ser speilveggen. = The dance partner sees the mirror wall.
• Dansepartneren ser på speilveggen. = The dance partner looks at the mirror wall.

Not exactly. På often means on, but prepositions do not match perfectly between English and Norwegian.

Here, på is part of the expression se på, which usually corresponds to English look at or watch.

So in this sentence:

• ser på = looks at

This is very common in Norwegian: a verb plus a preposition can have a meaning that you should learn as a unit.

Because Norwegian normally writes compound nouns as one word.

So:

• speil = mirror
• vegg = wall
• speilvegg = mirror wall

Then the definite ending is added:

• speilveggen = the mirror wall

This is very normal in Norwegian. English often writes similar ideas as two words, but Norwegian usually combines them.

Also, in compounds, the last part is the main noun. Here, vegg is the head, so the whole word behaves like a kind of wall.

The basic forms are:

• en dansepartner
• en speilvegg

Both are common gender nouns, which is why their definite singular forms end in -en:

• dansepartneren
• speilveggen

You may also find it helpful to see the pattern:

• en dansepartner → dansepartneren
• en speilvegg → speilveggen

This sentence has the normal word order for a simple Norwegian main clause:

• Dansepartneren = subject
• ser = finite verb
• på speilveggen = prepositional phrase

So the structure is basically:

Subject + Verb + Rest

That is the most neutral order in a statement like this.

A broader grammar point: Norwegian is a V2 language, which means the finite verb normally comes in the second position in main clauses. In this sentence, the subject comes first, so the verb naturally comes right after it.

Pronunciation varies by dialect, but a few useful points are:

• Stress usually falls early in the word, especially in compounds.
• Dansepartneren has main stress on the first part: DAN-separtneren
• Speilveggen has main stress on speil: SPEIL-veggen

A few sound notes:

• ser has a long vowel; it is not pronounced like English sir
• på has the Norwegian å sound, roughly similar to the vowel in British law
• speil is often pronounced roughly like spale in many varieties, though the exact sound depends on dialect

If you are learning Bokmål pronunciation, it is best to listen to native audio, since spelling-to-sound in Norwegian is usually fairly regular but still dialect-sensitive.