Learn Hardcore German: Der Mann, der im Garten sitzt, ist freundlich. - The man who is sitting in the garden is friendly.

The first Der in Der Mann is the definite article (the), marking Mann as masculine, singular, nominative.
The second der in der im Garten sitzt is a relative pronoun, meaning who/that and referring back to Der Mann.

So the structure is: Der Mann (the man), der (who) im Garten sitzt, ist freundlich.

What exactly is a “relative clause” in this sentence?

A relative clause gives extra information about a noun and is introduced by a relative pronoun (der, die, das, etc.).

In Der Mann, der im Garten sitzt, ist freundlich.

The main clause is: Der Mann ist freundlich.
The relative clause is: der im Garten sitzt – it tells you which man: the one who is sitting in the garden.

Relative clauses in German are almost always set off by commas.

Why is there a comma before and after der im Garten sitzt?

German always separates a relative clause from the rest of the sentence with commas.

First comma: after Der Mann, it marks the start of the relative clause.
Second comma: after sitzt, it marks the end of the relative clause.

So the commas show that der im Garten sitzt is an inserted description of Der Mann.

Why does the verb sitzt come at the end of the relative clause?

In German, main clauses normally have the finite verb in second position, but subordinate clauses (including relative clauses) have the finite verb at the end.

Main clause: Der Mann ist freundlich. (verb in 2nd position)
Relative clause: der im Garten sitzt (verb last)

So in a relative clause, the pattern is:
[relative pronoun] + [other elements] + [finite verb at the end].

Why is sitzt translated as “is sitting”? Why not use some continuous form in German?

Standard German does not have a special continuous/progressive form like English is sitting.

The simple present sitzt can mean:

he sits (habitual) or
he is sitting (right now), depending on context.

So der im Garten sitzt naturally corresponds to who is sitting in the garden in English.

Could I also say Der Mann im Garten ist freundlich? Is that the same?

Yes, Der Mann im Garten ist freundlich. is grammatical and natural, and it usually means the same in this context: The man in the garden is friendly.

The differences:

Der Mann, der im Garten sitzt, ist freundlich. uses a full relative clause and feels slightly more explicit and “complete.”
Der Mann im Garten uses im Garten as an attributive prepositional phrase directly after the noun.

In everyday speech, both are fine if it’s clear which man you mean.

Why is it im Garten and not in dem Garten?

im is simply the contracted form of in dem:

in + dem Garten → im Garten

This contraction is very common and usually preferred in spoken and written German unless you want to emphasize dem for some reason.

Why is Garten in the dative case here?

The preposition in is a two-way preposition (Wechselpräposition).
It takes:

Accusative for motion into something: in den Garten (into the garden)
Dative for location in something: im Garten (in the garden)

Here the man is simply located in the garden, not moving into it, so Garten is dative: im (in dem) Garten.

What case is Der Mann, and how can I tell?

Der Mann is in the nominative case, because Mann is the subject of the main clause.

It is the thing/person that is friendly.
The nominative masculine article is der.

If Mann were the direct object, you would see den Mann (accusative), and if it were an indirect object, dem Mann (dative).

Why is it just freundlich and not freundliche or freundlicher?

After the verb sein (to be), adjectives are used without endings (they are called predicate adjectives).

Der Mann ist freundlich.
Die Frau ist freundlich.
Die Kinder sind freundlich.

Adjective endings like -e, -er, -en appear when the adjective directly precedes a noun, e.g. ein freundlicher Mann, eine freundliche Frau, freundliche Kinder.

Could I use welcher instead of der in the relative clause?

In principle, yes:

Der Mann, welcher im Garten sitzt, ist freundlich.

This is grammatically correct but sounds formal, old-fashioned, or stylistic in modern German.
In everyday language, der/die/das is by far the most common relative pronoun for defining which person or thing you mean.

Why don’t we use wer instead of der here?

wer is not used like English who in normal relative clauses referring to a specific noun.

Der Mann, der im Garten sitzt, … → refers to a specific man.
Wer im Garten sitzt, ist freundlich. → literally: Whoever sits in the garden is friendly. (general statement, no specific noun)

So you use der/die/das (and their other forms) as relative pronouns that refer back to a particular noun; wer is more like whoever/anyone who.

Can the relative pronoun der be left out, like in English “The man sitting in the garden is friendly”?

No, you cannot simply drop der in a German relative clause:

❌ Der Mann, im Garten sitzt, ist freundlich. (wrong)
✅ Der Mann, der im Garten sitzt, ist freundlich.

If you want a structure closer to English “the man sitting in the garden”, you must change the grammar:

Der im Garten sitzende Mann ist freundlich. (participle construction, more written/formal)
Der Mann, der im Garten sitzt, ist freundlich. (normal relative clause)

In a standard relative clause, the relative pronoun must be present.