Hun åpner munnen.

Because munnen is the definite singular form of munn.

• en munn = a mouth
• munnen = the mouth

In this sentence, Norwegian uses the definite form, so munnen means the mouth.

With body parts, Norwegian very often uses the definite noun instead of a possessive when it is obvious whose body part is meant.

So Norwegian prefers:

• Hun åpner munnen.

rather than:

• Hun åpner hennes munn.

This sounds more natural in Norwegian. English usually says her mouth, but Norwegian often just says the mouth in these situations.

The base form is å åpne, which means to open.

• å = the infinitive marker, like to in English
• åpne = open

So:

• å åpne = to open
• åpner = opens / is opening

Åpner is the present tense.

For regular verbs like å åpne, the present tense is usually made by adding -r:

• å åpne → åpner

This form can mean both:

• she opens
• she is opening

Norwegian usually uses the same present-tense form for both ideas.

Because Norwegian normally does not need a separate progressive form like English does.

English distinguishes between:

• she opens
• she is opening

But Norwegian often uses just one form:

• Hun åpner

The context tells you whether it means a general action or something happening right now.

Hun is a subject pronoun meaning she.

It is used when the person is doing the action:

• Hun åpner munnen. = She opens/is opening the mouth.

Compare that with object forms in English:

• she = subject
• her = object

Norwegian hun corresponds to English she.

The sentence follows the basic Norwegian pattern:

• Subject + Verb + Object

So here:

• Hun = subject
• åpner = verb
• munnen = object

This is the standard word order for a simple statement.

Yes. Norwegian is a V2 language, which means the finite verb usually comes in the second position in main clauses.

So if you start with another element, the verb still stays second:

• Hun åpner munnen.
• Nå åpner hun munnen.

Notice that after Nå, the verb åpner comes before hun.

This is very important in Norwegian sentence structure.

Munn is usually masculine in Bokmål.

So its forms are:

• en munn = a mouth
• munnen = the mouth

That -en ending is the normal definite singular ending for many masculine nouns.

The letter å is a separate letter in Norwegian, not just a with a mark.

It is pronounced roughly like the vowel in English more or saw, though the exact sound depends on dialect.

So åpner starts with a broad aw sound.

A rough guide for an English speaker could be:

• Hun ≈ huun with a rounded vowel
• åpner ≈ AWP-ner
• munnen ≈ MUN-nen

A few notes:

• The u in hun is not exactly like English oo, but that is a useful approximation.
• The å in åpner is the most important sound to notice.
• The r may sound different depending on dialect.

You could form that sentence, but it sounds unnatural in ordinary Norwegian.

Hennes is used for her, but with body parts Norwegian usually avoids it when the owner is already clear.

So:

• Hun åpner munnen. = natural
• Hun åpner hennes munn. = grammatical in some contexts, but usually odd

Using hennes would make more sense only if you really needed to emphasize whose mouth it is.

Yes, but it sounds more formal, literary, or emphatic than the normal version.

• sin can mean her own
• Hun åpner sin munn has a slightly marked feel, almost like She opens her own mouth

In everyday Norwegian, Hun åpner munnen is the natural choice.

Because the definite form of munn is built by adding the definite ending to the noun stem:

• munn
  • -en → munnen

The spelling keeps the double n from the original word.

This is normal Norwegian spelling, not a special exception for this sentence.

This sentence is standard Bokmål.

In Nynorsk, you would more commonly see:

• Ho opnar munnen.

So the differences include:

• Hun → Ho
• åpner → opnar

The sentence you were given is clearly Bokmål.