Learn Hardcore German: Der, der im Garten sitzt, ist mein Bruder. - The one who is sitting in the garden is my brother.

The first Der is a demonstrative pronoun: roughly “that one / the one”.
The second der is a relative pronoun: like “who / that” in English.

So the structure is basically: That one, who is sitting in the garden, is my brother.
It looks like “der, der …”, but it’s really “that one, who …” in German form.

What exactly is the job of the first Der?

The first Der is a pronoun standing alone, not an article before a noun.

If it were an article, you would expect a noun after it, like der Mann (the man).
Here it stands by itself, so it’s a demonstrative pronoun, similar to “that one” or “the one” in English.

It refers to a specific person already known from the context (for example: there may be several people, and you are picking out “that one”).

What is the second der doing?

The second der is a relative pronoun that introduces a relative clause: der im Garten sitzt.

It refers back to the first Der (same person).
It means roughly “who” or “that” in English.
It is the subject of the verb sitzt inside the relative clause.

So: Der (that one), der (who) im Garten sitzt, ist mein Bruder.

Why is the second der in the form der and not den or dem?

Because inside the relative clause der im Garten sitzt, the pronoun is the subject of sitzt.

Subject → nominative case → masculine singular form = der.
If it were an object, the form would change, for example:
- Der, den du im Garten siehst, ist mein Bruder.
  - Here den is accusative, because it’s the object of siehst.

So the case of the relative der is determined by its role inside its own clause, not by the main clause.

Why are there commas around der im Garten sitzt?

German always separates a full relative clause with commas.

The relative clause is der im Garten sitzt.
It is inserted into the main clause and must be set off by commas on both sides:
- Der, der im Garten sitzt, ist mein Bruder.

This is not optional in standard written German; leaving out either comma would be a spelling/grammar mistake.

Why does the verb sitzt come at the very end of der im Garten sitzt?

Because der im Garten sitzt is a subordinate clause (a relative clause), and in German:

In subordinate clauses, the finite verb (here sitzt) goes to the end.
In main clauses, the finite verb is in the second position.

Compare:

Main clause: Er sitzt im Garten. (verb in second position)
Relative clause: … der im Garten sitzt. (same words, but verb moved to the end)

Can I say Der im Garten sitzt, ist mein Bruder without the second der?

Yes, that is possible in German and quite common, especially in spoken language:

Der im Garten sitzt, ist mein Bruder.

Here der im Garten sitzt functions like “the one who is sitting in the garden”.
The version with der, der … is a bit more explicit and slightly more formal/emphatic.

So both are grammatical; your original sentence just uses a clearer, fully spelled‑out relative pronoun.

Could I use wer instead of the second der, like Der, wer im Garten sitzt, ist mein Bruder?

No, Der, wer … is wrong in standard German.

wer is used without a specific antecedent and often means “whoever / anyone who”, e.g.:
- Wer im Garten sitzt, ist mein Bruder. → “Whoever is sitting in the garden is my brother.” / “The one who is sitting in the garden is my brother.”

Once you have a clear antecedent (Der …), you must use a der/die/das‑type relative pronoun, not wer.
That is why the correct form is Der, der im Garten sitzt, ist mein Bruder.

Could I replace the relative der with welcher, as in Der, welcher im Garten sitzt, ist mein Bruder?

Yes, that is grammatically possible:

Der, welcher im Garten sitzt, ist mein Bruder.

However:

welcher as a relative pronoun sounds formal, old‑fashioned, or bookish in modern German.
In everyday speech and writing, der/die/das is far more common.

So you can say it, but Der, der im Garten sitzt, … is the normal, natural choice.

What does im mean exactly, and why not in dem?

im is simply the contracted form of in dem:

in (in) + dem (the; dative masculine/neuter) → im.
Garten is masculine, and after in (location) you need dative, so in dem Garten → im Garten.

The contraction im is standard and much more common than the full in dem in this context.

Why is Garten capitalized?

In German, all nouns are capitalized, regardless of their position in the sentence.

Garten is a noun (a thing/place), so it must start with a capital letter: Garten.
This is a general rule: Haus, Auto, Freund, Bruder, Garten, etc. are all written with capital initial letters.

Why is it mein Bruder and not meinen Bruder or meinem Bruder?

Because Bruder is in the nominative case, not accusative or dative.

The whole structure is: [Subject] ist [predicate noun].
In German, both sides of sein (ist) are in the nominative:
- Der … ist mein Bruder.

The possessive determiner mein follows the same pattern as ein:

nominative masculine singular → mein Bruder (no extra ending).
You would only see meinen Bruder (accusative) or meinem Bruder (dative) if the verb or preposition required those cases, which ist does not.