Min arm gør ondt, fordi jeg bar den tunge taske i går.

Because arm is a common gender noun in Danish.

• en arm = an arm
• Common gender nouns take min, din, den, etc.
• Neuter nouns take mit, dit, det, etc.

So:

• min arm = my arm
• but mit ben = my leg

A good habit is to learn Danish nouns together with their article: en arm, en taske.

In Danish, a possessive like min usually replaces the article.

So you say:

• min arm = my arm
• not den min arm

This works like English: we say my arm, not the my arm.

Gøre ondt is a very common fixed expression meaning to hurt or to be painful.

In this sentence:

• Min arm gør ondt = My arm hurts

Literally, it is something like does painful, but you should learn it as a set phrase, not word by word.

You will often see it in patterns like:

• Det gør ondt = It hurts
• Det gør ondt i armen = It hurts in the arm
• Min arm gør ondt = My arm hurts

Because bære is an irregular verb.

Its main forms are:

• at bære = to carry
• bærer = carries / is carrying
• bar = carried
• båret = carried

So:

• jeg bar = I carried

This is something you simply have to memorize, just like irregular verbs in English such as go → went.

This structure is used for a specific/definite noun with an adjective.

Pattern:

• den + adjective + noun

So:

• den tunge taske = the heavy bag

Here is how it compares:

• en tung taske = a heavy bag
• den tunge taske = the heavy bag

The sentence uses den tunge taske because it refers to a particular bag, not just any heavy bag.

Because adjectives usually take -e when they come before a definite noun.

Compare:

• en tung taske = a heavy bag
• den tunge taske = the heavy bag

So the -e is not random; it is part of the adjective form used in definite noun phrases.

Because in this pattern, Danish uses den + adjective + noun with the noun in its basic form.

So the correct form is:

• den tunge taske

Not:

• den tunge tasken

A useful comparison:

• tasken = the bag
• den tunge taske = the heavy bag

When there is no adjective, Danish often uses the ending:

• tasken

But when there is an adjective before the noun, Danish normally uses:

• den tunge taske

Because time expressions often come later in the clause, especially after the object.

So this is a very normal word order:

• jeg bar den tunge taske i går

But i går can also move, depending on what you want to emphasize:

• I går bar jeg den tunge taske = Yesterday I carried the heavy bag

Both are correct. The original sentence just uses a neutral, common placement.

Yes. Fordi introduces a subordinate clause, and subordinate clauses in Danish follow different word-order rules from main clauses.

In this sentence, you see:

• fordi jeg bar den tunge taske i går

Here, the word order looks similar to English. But the difference becomes clearer when there is an adverb like ikke:

• fordi jeg ikke bar den tunge taske
• not fordi bar jeg ikke den tunge taske

So after fordi, Danish does not use normal main-clause word order.

Yes, absolutely. That is also very natural Danish.

The difference is mostly one of perspective:

• Min arm gør ondt = My arm hurts
• Jeg har ondt i armen = I have pain in my arm / My arm hurts

The first makes the arm the subject.
The second makes you the subject and says where the pain is.

Both are common, and both are worth learning.