Learn Hardcore German: Das Feuer im Kamin macht das Wohnzimmer gemütlich. - The fire in the fireplace makes the living room cozy.

Das Feuer im Kamin = subject (nominative)
macht = verb
das Wohnzimmer = direct object (accusative)
gemütlich = object complement (describes the resulting state of das Wohnzimmer)

So literally: The fire in the fireplace makes the living room cozy.

You can see the subject and object more clearly if you simplify:

Das Feuer macht das Wohnzimmer gemütlich.
→ Das Feuer = what does something (subject)
→ das Wohnzimmer = what is affected/changed (object)

Why is it im Kamin and not in den Kamin?

The choice between im Kamin and in den Kamin depends on the case, which is controlled by the preposition in:

in
- dative = location (where something is)
in
- accusative = direction (where something is going)

In this sentence, the fire is located in the fireplace, so you use the dative:

im Kamin = in dem Kamin (in the fireplace) → dative singular, masculine

If you used in den Kamin, that would be accusative and mean:

in den Kamin = into the fireplace (movement into it)

Example contrast:

Das Feuer ist im Kamin. – The fire is in the fireplace. (location → dative)
Er legt Holz in den Kamin. – He puts wood into the fireplace. (direction → accusative)

What exactly does macht … gemütlich mean? Why not just say ist gemütlich?

There is an important difference:

Das Wohnzimmer ist gemütlich.
→ The living room is cozy. (a simple description)
Das Feuer im Kamin macht das Wohnzimmer gemütlich.
→ The fire makes the living room cozy. (it causes it to be cozy)

So machen + object + adjective is a causative structure:

Das Feuer macht das Wohnzimmer gemütlich.
Die Musik macht mich glücklich. – The music makes me happy.
Das Licht macht den Raum hell. – The light makes the room bright.

If you said:

Das Feuer im Kamin ist gemütlich.

that would focus on the fire itself being cozy, not on the living room becoming cozy because of it. The original sentence emphasizes the effect of the fire on the room.

What does gemütlich really mean? Is it just “cozy”?

Gemütlich is often translated as cozy, but it has a slightly broader feel:

physically cozy: warm, comfortable, pleasant
emotionally cozy: relaxed, homely, friendly atmosphere

It can describe:

a place: ein gemütliches Wohnzimmer – a cozy living room
a situation: ein gemütlicher Abend – a relaxed, cozy evening
furniture/lighting: ein gemütliches Sofa, gemütliches Licht

It’s close to the idea of a warm, inviting, comforting atmosphere—not just soft cushions, but also a sense of relaxation and comfort.

Why do we have das Feuer, im Kamin, and das Wohnzimmer? What are the genders and cases here?

The three nouns are:

das Feuer – neuter, nominative (subject)
der Kamin → im Kamin – masculine, dative (location)
das Wohnzimmer – neuter, accusative (direct object)

Details:

das Feuer
- Gender: neuter (das)
- Case: nominative (subject of macht)
im Kamin
- im is a contraction of in dem
- Kamin is masculine (der Kamin)
- Case: dative (dem Kamin) because in expresses location here
das Wohnzimmer
- Gender: neuter (das)
- Case: accusative (it is what is being made cozy)

You can see the article patterns:

Masculine: der (Nom.), dem (Dat.), den (Acc.)
Neuter: das (Nom.), dem (Dat.), das (Acc.)

In this sentence you have:

das (neuter nominative) – das Feuer
dem (masculine dative) – hidden inside im Kamin
das (neuter accusative) – das Wohnzimmer

What exactly is im? How is it formed?

im is a standard contraction of:

in + dem → im

You use it when in is followed by a masculine or neuter noun in the dative singular:

in dem Kamin → im Kamin
in dem Zimmer → im Zimmer
in dem Haus → im Haus

This is very common in spoken and written German. Other frequent contractions:

an dem → am (e.g. am Tisch)
bei dem → beim
zu dem → zum
zu der → zur

Could I say Das Feuer macht das Wohnzimmer im Kamin gemütlich? Is that word order possible?

Grammatically, the word order is possible, but the meaning changes and sounds strange:

Das Feuer im Kamin macht das Wohnzimmer gemütlich.
→ The fire (which is in the fireplace) makes the living room cozy.
Das Feuer macht das Wohnzimmer im Kamin gemütlich.
→ The fire makes the living room in the fireplace cozy.
(This sounds odd because you normally don’t have a living room in a fireplace.)

In German, adverbials and prepositional phrases are somewhat flexible, but their position can strongly affect what they are understood to modify.

im Kamin right after das Feuer → describes the fire
im Kamin right after das Wohnzimmer → describes the living room

Correct and natural is the original version, where im Kamin clearly belongs to das Feuer.

If I add adjectives, how do the endings work in a sentence like this?

Let’s extend the sentence:

Das warme Feuer im alten Kamin macht das kleine Wohnzimmer sehr gemütlich.

Breakdown of adjective endings:

das warme Feuer
- Feuer: neuter, nominative, with definite article das
- Adjective ending: -e → warme
im alten Kamin
- Kamin: masculine, dative, with definite article dem inside im
- Adjective ending: -en → alten
das kleine Wohnzimmer
- Wohnzimmer: neuter, accusative, with definite article das
- Adjective ending: -e → kleine

Rule here (with definite article in singular):

Nominative: der/die/das → adjective takes -e
Accusative: same for neuter and feminine (das/die) → -e
Dative (all genders): adjective takes -en

So: das warme Feuer, im alten Kamin, das kleine Wohnzimmer.

How would I say this sentence in the past or future tense?

Present (original):

Das Feuer im Kamin macht das Wohnzimmer gemütlich.

Perfect tense (spoken past):

Das Feuer im Kamin hat das Wohnzimmer gemütlich gemacht.
(The fire in the fireplace made the living room cozy.)

Präteritum (simple past, more written):

Das Feuer im Kamin machte das Wohnzimmer gemütlich.

Future:

Das Feuer im Kamin wird das Wohnzimmer gemütlich machen.
(The fire in the fireplace will make the living room cozy.)

Patterns:

machen → hat gemacht (perfect)
machen → machte (Präteritum)
werden + machen for future: wird … machen

How would the sentence change if there were several fires or several fireplaces?

With plural fireplaces:

Das Feuer in den Kaminen macht das Wohnzimmer gemütlich.
(The fire in the fireplaces makes the living room cozy.)
– less common in reality, but grammatically correct

Here in den Kaminen:

die Kamine (plural)
in + dative plural → in den Kaminen

With several fires making it cozy (more abstract):

Die Feuer im Kamin machen das Wohnzimmer gemütlich.

Changes:

Die Feuer (plural subject)
Verb must agree: machen (not macht)

So:

Singular: Das Feuer … macht …
Plural: Die Feuer … machen …