Jeg hænger frakken bag døren, når jeg kommer hjem.

Hænger is the present tense form of the verb at hænge.

• at hænge = to hang
• jeg hænger = I hang / I am hanging

In Danish, the present tense is often used in sentences about:

• habits and routines
• general truths
• sometimes the near future

So in this sentence, jeg hænger frakken bag døren means something like I hang the coat behind the door as a regular action.

Usually it sounds like a habitual action: something the speaker normally does whenever they get home.

That is because:

• når often introduces something that happens whenever or when
• both verbs are in the present tense

So the sentence most naturally suggests:

• Whenever I come home, I hang the coat behind the door

Depending on context, present tense in Danish can also refer to the future, but here the routine/habit reading is the most natural one.

Frakken means the coat. Danish often uses the definite form where English might use:

• the coat
• my coat
• his coat, etc.

If it is obvious whose coat is meant, Danish does not always need a possessive word like min.

So:

• frakken = the coat
• in context, it may naturally mean my coat

If you said min frakke, that would be more explicitly my coat. Both can be possible, but frakken sounds very natural when the ownership is already understood.

Because bag døren means behind the door, referring to a specific door.

Compare:

• en dør = a door
• døren = the door

In this sentence, the speaker probably means a particular door in the house, so the definite form is used.

Also note that after prepositions, Danish still uses the definite form when English would use the:

• bag døren = behind the door
• på bordet = on the table
• i bilen = in the car

Bag means behind. It is the normal word when something is located on the other side of something else.

So:

• bag døren = behind the door

You may also see bagved, which is very close in meaning:

• bagved døren = behind the door

In many contexts, bag and bagved are both possible, though bag is very common and simple here.

Because når jeg kommer hjem is a subordinate clause, and Danish word order changes in subordinate clauses.

In a main clause, Danish often has verb-second word order:

• Jeg kommer hjem
• Nu kommer jeg hjem

But after a subordinating word like når, the subject usually comes before the finite verb:

• når jeg kommer hjem

So:

• når jeg kommer hjem = correct
• når kommer jeg hjem would sound like a direct question: When am I coming home?

Because når jeg kommer hjem is a subordinate clause.

In Danish, commas are often used to separate subordinate clauses from the main clause:

• Jeg hænger frakken bag døren, når jeg kommer hjem.

However, learners should know that Danish comma usage has had more than one accepted system. So you may also see sentences without that comma in some styles.

Still, the comma here is completely normal and easy to understand:

• main clause: Jeg hænger frakken bag døren
• subordinate clause: når jeg kommer hjem

In Danish, hjem means homeward / home as a direction. It is used without a preposition here.

So:

• jeg kommer hjem = I come home / I get home

Compare:

• hjem = motion toward home
• hjemme = being at home

Examples:

• Jeg går hjem. = I walk home.
• Jeg er hjemme. = I am at home.

So kommer hjem is correct because the sentence is about going/coming to home, not already being there.

No, and this is a very common point for learners.

In time clauses:

• når is usually used for repeated events, general situations, or future time
• da is usually used for one specific event in the past

So here, når is correct because the meaning is something like whenever I come home.

Compare:

• Når jeg kommer hjem, hænger jeg frakken bag døren. = Whenever/When I come home, I hang the coat behind the door.
• Da jeg kom hjem, hang jeg frakken bag døren. = When I came home, I hung the coat behind the door.

Yes. Danish allows that very naturally.

You can say:

• Jeg hænger frakken bag døren, når jeg kommer hjem.
• Når jeg kommer hjem, hænger jeg frakken bag døren.

Both are correct.

If the subordinate clause comes first, the main clause must still follow normal Danish main-clause word order, which means the finite verb comes before the subject in the rest of the clause:

• Når jeg kommer hjem, hænger jeg frakken bag døren.

Notice:

• not Når jeg kommer hjem, jeg hænger ...
• but Når jeg kommer hjem, hænger jeg ...

That is a very important Danish word-order pattern.