Hendes bagage er tungere end min, fordi hun skal blive der hele ugen.

Because sin/sit/sine is a reflexive possessive: it refers back to the subject of the same clause.

In Hendes bagage er tungere ..., the subject is bagage, not hun. So Danish cannot use sin here. You need hendes = her.

Compare:

• Hun pakker sin bagage. = She packs her own luggage.
• Hendes bagage er tungere end min. = Her luggage is heavier than mine.

So hendes is correct because the owner is not the clause subject.

Bagage is usually a mass noun, like English luggage. Even if it refers to several bags, Danish normally treats it as one uncountable thing.

That is why the sentence uses er:

• Hendes bagage er tungere ...

Not plural.

You usually would not say bagager in ordinary everyday Danish.

Tungere is the comparative of tung = heavy.

The pattern is:

• tung = heavy
• tungere = heavier
• tungest = heaviest

And end means than:

• tungere end min = heavier than mine

So the structure is very similar to English:

• adjective in comparative form + end

Because min agrees with the understood noun, which is bagage.

Here min means my luggage / mine. The noun is omitted because it is already clear from context.

The forms are:

• min for common gender singular
• mit for neuter singular
• mine for plural

Since bagage is common gender singular, the correct form is min.

So:

• min bagage = my luggage
• hendes bagage er tungere end min = her luggage is heavier than mine

Because the comparison is between two sets of luggage, not between two people.

Min here stands for min bagage.

So:

• hendes bagage ... end min = her luggage ... than mine

If you said end mig, that would mean than me, which does not match the idea of comparing luggage.

This works like English:

• heavier than mine not
• heavier than me

Here skal means something like has to, needs to, or is supposed to.

So:

• hun skal blive der hele ugen = she has to stay there all week

In modern Danish, skal often expresses obligation, necessity, or a planned/expected action, not just future time.

Blive has more than one meaning in Danish. Two very common ones are:

• become
• stay / remain

In this sentence, because it is followed by der = there, it means stay:

• blive der = stay there

So:

• hun skal blive der hele ugen = she has to stay there all week

With time expressions like day, week, month, year, Danish often uses hel/hele + definite form:

• hele dagen = all day / the whole day
• hele ugen = all week / the whole week
• hele året = all year

So hele ugen is the normal idiomatic form here.

Hele uge would sound incomplete or unnatural in this sentence.

Here der simply means there, referring to a place.

So:

• blive der = stay there

This is the normal location word, not the other Danish der that can act like a dummy subject in sentences such as Der er mange mennesker her.

Yes. Fordi introduces a subordinate clause.

In this sentence, the word order looks normal because there is no sentence adverb like ikke, but it is still a subordinate clause:

• fordi hun skal blive der hele ugen

A clearer comparison is:

• Main clause: Hun skal ikke blive der.
• Subordinate clause: ... fordi hun ikke skal blive der.

So after fordi, standard Danish uses subordinate-clause word order.

Because it is a time expression, and Danish often places time expressions late in the clause, especially after the verb phrase and place expression.

So this order is very natural:

• hun skal blive der hele ugen

Literally:

• she has to stay there all week

You could think of the structure as:

• subject: hun
• finite verb: skal
• infinitive: blive
• place: der
• time: hele ugen

In normal Danish, no. With an adjective like tung, you use the -ere comparative:

• tungere

So the natural form is:

• Hendes bagage er tungere end min

Using mere is more typical with many longer adjectives, but not with a short common adjective like tung.