Det er roen i haven, der hjælper mig med at slappe af efter arbejdet.

This is a very common Danish cleft sentence pattern. It is used to highlight or emphasize one part of the sentence.

• Det er roen i haven, der hjælper mig ... = It is the peace in the garden that helps me ...

Without the cleft structure, the sentence could simply be:

• Roen i haven hjælper mig med at slappe af efter arbejdet.

That version is more neutral.
The Det er ... der ... version puts extra focus on roen i haven.

Because roen is the definite form of ro.

• ro = calm / peace / quiet
• roen = the calm / the peace / the quiet

In this sentence, Danish is talking about a specific calmness: the peace in the garden. So the definite form is natural:

• roen i haven = the peace in the garden

This is different from English, where we often leave abstract nouns without the. Danish uses the definite form more often in cases like this.

Here, roen means something like:

• the peace
• the quiet
• the calm

It refers to the restful, peaceful atmosphere of the garden.
It does not mean physical stillness only; it can also describe a mental sense of calm.

So roen i haven is best understood as the peacefulness of the garden or the peace in the garden.

Because i haven means in the garden, and that is the natural way to describe where the peace is found.

• roen i haven = the peace in the garden

Using af would suggest something more like of the garden in a possessive or descriptive sense, and that is not the normal phrasing here.

Danish often uses i where English might sometimes choose a more flexible phrase like of or in depending on style. Here i haven is the idiomatic choice.

In this kind of sentence, Danish usually uses der as the relative word when it is the subject of the relative clause.

Here, der refers back to roen i haven, and it is the thing that does the helping:

• det er roen i haven, der hjælper mig ...
• literally: it is the peace in the garden that helps me ...

Because der is the subject of hjælper, it is the normal choice.

A useful rule of thumb:

• Use der when the relative word is the subject
• Use som less often in modern Danish in this kind of structure, especially after cleft sentences like this one

So der is the expected and natural word here.

Because Danish writes commas before subordinate and relative clauses much more regularly than English does.

In this sentence:

• Det er roen i haven, der hjælper mig ...

the part beginning with der is a relative clause, so it is separated by a comma.

This can feel unusual to English speakers, because English punctuation is often lighter. But in Danish, this comma is standard.

The verb hjælpe can be followed by different patterns, and one common one is:

• hjælpe nogen med at + infinitive

So:

• hjælper mig med at slappe af = helps me to relax

Here is the structure:

• hjælper = helps
• mig = me
• med at slappe af = with relaxing / to relax

Danish often uses med at where English simply uses to.

You may also see:

• hjælpe nogen til at ...

That can also mean help someone to ..., but med at is very natural here.

Because slappe af is the full verb expression meaning to relax.

• slappe af = relax

The word af is part of the verb, not a separate optional word.
This is similar to a phrasal verb in English.

Examples:

• Jeg slapper af. = I relax / I am relaxing.
• Det hjælper mig med at slappe af. = It helps me relax.

So if you leave out af, the meaning is incomplete or unnatural.

Because slappe af is a separable verb expression, and in the infinitive it normally appears together as:

• at slappe af

In a finite clause, Danish often separates the main verb and the particle:

• Jeg slapper af.

But in the infinitive, the form is typically:

• at slappe af

So this sentence uses the normal infinitive form after med at.

In Danish, when talking about after work in the sense of after the working day / after my work, the definite form arbejdet is very common.

• arbejde = work
• arbejdet = the work

So:

• efter arbejdet = after work

This is one of those places where Danish often uses a definite form where English does not.

Compare:

• efter arbejdet = after work
• på arbejdet = at work

These fixed expressions are worth learning as chunks.

Yes. A simpler version would be:

• Roen i haven hjælper mig med at slappe af efter arbejdet.

This means essentially the same thing, but it is less emphatic.

Compare:

• Roen i haven hjælper mig ... = neutral statement

• Det er roen i haven, der hjælper mig ... = emphasizes that it is specifically the peace in the garden that helps

So the original sentence is not about different basic meaning; it is about focus.

Because the part after der is a relative clause, and Danish subordinate clauses usually follow standard clause structure without the inversion you often see in main clauses.

Here:

• der hjælper mig med at slappe af efter arbejdet

the order is:

• der = subject marker referring back to roen i haven
• hjælper = verb
• mig = object
• med at slappe af = complement
• efter arbejdet = time expression

There is nothing unusual here once you recognize der as introducing the relative clause.

Yes. Have means garden, and haven means the garden.

• have = garden
• haven = the garden

So:

• i haven = in the garden

Danish usually adds the definite article as an ending:

• en have = a garden
• haven = the garden

That is why both roen and haven have endings showing definiteness.

In this sentence, der is best translated as that:

• the peace in the garden that helps me relax

But depending on context, Danish der in relative clauses can correspond to English:

• who
• which
• that

Here it refers to roen, so English would normally use that or which.
Because the sentence is a cleft sentence, that sounds especially natural in English:

• It is the peace in the garden that helps me relax after work.

Yes. The verb hjælper is present tense:

• hjælper = helps

This suggests a general or repeated situation:

• the peace in the garden helps me relax after work
• this is something that is generally true, not just happening once

So the sentence describes a habitual effect, not a single completed event.

A very useful way to learn Danish is to memorize common chunks. From this sentence, good ones are:

• Det er ..., der ... = It is ... that ...
• roen i haven = the peace in the garden
• hjælpe nogen med at ... = help someone to ...
• slappe af = relax
• efter arbejdet = after work

If you learn these as whole pieces, the sentence becomes much easier to understand and reuse.