Når jeg er færdig i haven, sætter jeg gødningen tilbage i garagen.

Når means when here.

It introduces a time clause: Når jeg er færdig i haven = When I’m finished in the garden / when I’m done gardening.

A learner often compares når with da:

• når is used for something that happens regularly, generally, or in the future
• da is usually used for a specific event in the past

So in this sentence, når is correct because it describes what happens whenever that situation occurs.

This is because Danish main clauses normally follow the verb-second rule.

The first part, Når jeg er færdig i haven, is a subordinate clause. When that whole clause comes first, it takes the first position in the sentence. Then the finite verb of the main clause must come next:

• Når jeg er færdig i haven, sætter jeg gødningen tilbage i garagen.

So the order is:

1. fronted subordinate clause
2. finite verb: sætter
3. subject: jeg

If the main clause came first, you would say:

• Jeg sætter gødningen tilbage i garagen, når jeg er færdig i haven.

Er færdig means am finished or am done.

It is very common in Danish to use være færdig to talk about finishing an activity:

• Jeg er færdig = I’m done
• Jeg er færdig med arbejdet = I’m finished with the work

In your sentence, jeg er færdig i haven means something like I’m finished out in the garden or I’m done working in the garden.

I haven literally means in the garden, but in Danish it often refers naturally to doing something there.

So:

• jeg er færdig i haven = I’m done in the garden

This sounds natural if the meaning is I’m finished with my work in the garden.

If you want to be more explicit about the activity, Danish could also say:

• jeg er færdig med havearbejdet = I’m finished with the garden work

So i haven focuses on the place, while med havearbejdet focuses on the task.

Sætter ... tilbage means put back.

Here:

• sætter = put / place / set
• tilbage = back

Together they mean returning something to where it belongs:

• sætte noget tilbage = to put something back

So:

• sætter jeg gødningen tilbage i garagen = I put the fertilizer back in the garage

Because gødningen is the definite form: the fertilizer.

In Danish, the definite article is often added to the end of the noun:

• gødning = fertilizer
• gødningen = the fertilizer

This sentence is talking about a specific fertilizer that the speaker has been using, so the definite form makes sense.

For the same reason: garagen is the definite form, meaning the garage.

• garage = a garage
• garagen = the garage

So:

• i garagen = in the garage

Danish often attaches the definite article to the end of the noun instead of using a separate word like English the.

I usually means in.

In this sentence:

• i haven = in the garden
• i garagen = in the garage

For garagen, this matches English very closely.

For haven, Danish uses i where English sometimes prefers in and sometimes just talks about the activity, such as working in the garden. So it is a normal Danish choice here.

Grammatically, it uses the present tense, but Danish present tense can cover several meanings depending on context.

This sentence could mean:

• a habitual action: Whenever I’m done in the garden, I put the fertilizer back in the garage
• a future action: When I’m done in the garden, I’ll put the fertilizer back in the garage

That is very normal in Danish. English often uses the present in the when clause too, but may use will in the main clause. Danish does not need a separate future form here.

Yes. That is perfectly natural if the context already makes it clear what you are finished with.

Adding i haven simply makes the situation more specific.

Compare:

• Når jeg er færdig, sætter jeg gødningen tilbage i garagen.
  = When I’m done, I put the fertilizer back in the garage.

• Når jeg er færdig i haven, sætter jeg gødningen tilbage i garagen.
  = When I’m done in the garden, I put the fertilizer back in the garage.

Yes, but the version in your sentence is very natural.

The common pattern is:

• sætte noget tilbage et sted

So:

• sætter jeg gødningen tilbage i garagen

sounds good and clear.

Danish word order can sometimes vary a little with adverbs and particles, but in this sentence tilbage fits very naturally before i garagen.

It looks long because it contains the noun plus the definite ending:

• gødning = fertilizer
• gødningen = the fertilizer

The rough breakdown is:

• gød-ning-en

A very approximate English-style guide would be something like GURTH-ning-en, but that is only rough. The vowel ø does not exist in English, so it takes practice.

The important thing for a learner is to recognize the parts:

• base noun: gødning
• definite ending: -en

Yes: the most typically Danish feature here is the verb-second word order in the main clause.

A native English speaker might expect:

• When I am done in the garden, I put the fertilizer back in the garage

In Danish, after the opening Når-clause, the main verb comes before the subject:

• Når jeg er færdig i haven, sætter jeg gødningen tilbage i garagen.

That inversion is one of the most important word-order patterns to learn in Danish.