Hun løfter tasken forkert, og derfor bliver hendes skulder og ryg hurtigt ømme.

Because løfter is the present tense of at løfte.

• at løfte = to lift
• hun løfter = she lifts / she is lifting

A useful thing for English speakers: Danish verbs do not change by person in the present tense.

• jeg løfter
• du løfter
• hun løfter
• vi løfter

So the -r here is just the normal present-tense ending.

In Danish, the definite article is often attached to the end of the noun.

• en taske = a bag
• tasken = the bag

So instead of a separate word like English the, Danish often adds -en, -n, -et, or -t to the noun, depending on the noun.

Here:

• taske = bag
• tasken = the bag

Here forkert means wrongly or incorrectly.

In this sentence it describes how she lifts the bag, so it works like an adverb in English:

• Hun løfter tasken forkert = She lifts the bag incorrectly

A learner might expect a separate adverb form, but Danish often uses adjective forms this way. So forkert can mean both wrong and wrongly/incorrectly, depending on the sentence.

Because the sentence contains two main clauses joined by og:

1. Hun løfter tasken forkert
2. derfor bliver hendes skulder og ryg hurtigt ømme

Each clause has its own finite verb:

• løfter
• bliver

Danish commonly uses a comma between coordinated main clauses, even with og. This is often more regular than in English, where a comma before and is less consistent.

Because Danish main clauses follow the verb-second rule.

That means the finite verb usually comes in the second position of the clause.

In the second clause, derfor comes first, so the verb must come next:

• derfor bliver hendes skulder og ryg hurtigt ømme

Not:

• derfor hendes skulder og ryg bliver ...

A helpful way to see it:

• first element: derfor
• second element: bliver
• then the subject: hendes skulder og ryg

Also, og does not count as the first element here. The real first element is derfor.

Because bliver means becomes, not just is.

• er øm/ømme = is/are sore
• bliver øm/ømme = becomes/become sore

So the sentence describes a change of state:

• first she lifts the bag the wrong way
• as a result, her shoulder and back become sore

If you said er, it would describe the state only, not the process of becoming sore.

This is a very good question, because English speakers often learn that Danish uses sin/sit/sine for his/her own.

Here, though, hendes is correct because of the clause structure.

In the second clause:

• derfor bliver hendes skulder og ryg hurtigt ømme

the subject is hendes skulder og ryg.

Reflexive possessives like sin/sit/sine normally refer to the subject of their own clause, and they are typically used when the possessed thing is not itself the subject phrase. In this sentence, the body parts are the subject of the clause, so Danish uses hendes.

So even though the shoulder and back belong to hun, you still say:

• hendes skulder og ryg

not:

• sin skulder og ryg

A useful comparison:

• Hun løfter tasken forkert og skader sin ryg.
  Here sin works, because hun is the subject of the same clause and sin ryg is the object.

Because possessives replace the article.

Just like in English you say:

• her shoulder
• not the her shoulder

Danish does the same:

• hendes skulder
• not den hendes skulder

So hendes already does the job of marking possession, and no extra definite article is needed.

Because hurtigt is the adverb form of hurtig.

• hurtig = quick
• hurtigt = quickly

Here it describes how fast the soreness develops:

• bliver hurtigt ømme = become sore quickly

This -t pattern is very common in Danish:

• langsom → langsomt
• rigtig → rigtigt
• hurtig → hurtigt

Because it modifies the whole idea of becoming sore, not just the adjective by itself.

• bliver hurtigt ømme = become sore quickly

This word order is very natural in Danish. After the verb and subject, adverbs like hurtigt often come before the predicative adjective:

• Han bliver hurtigt træt = He gets tired quickly
• De bliver hurtigt vrede = They get angry quickly

So hurtigt is in a normal Danish position here.

Because the adjective agrees with a plural subject.

The subject is:

• hendes skulder og ryg

Even though each noun is singular, together they form a combined subject: shoulder and back = two things.

So Danish uses the plural form of the adjective:

• øm = sore (singular)
• ømme = sore (plural)

Compare:

• Hendes ryg bliver øm = Her back becomes sore
• Hendes skulder og ryg bliver ømme = Her shoulder and back become sore

Yes, Danish could say skuldre if both shoulders were meant. Here skulder is singular because the sentence is talking about one shoulder and the back.

That makes good sense in context, since carrying or lifting a bag badly may affect one shoulder more than the other.

So:

• skulder = shoulder
• skuldre = shoulders

Both are possible in Danish, depending on the exact meaning.