Learn Hardcore German: Ich stelle den Stuhl gegen die Wand.

Because Stuhl is masculine and it’s the direct object (accusative). The definite article for masculine accusative is den. With an indefinite article it would be einen Stuhl.

What cases are used in the sentence?

den Stuhl: accusative as the direct object of stellen.
gegen die Wand: accusative because the preposition gegen always takes the accusative, and it indicates movement toward contact with the wall. (Feminine accusative is also die, so it doesn’t change form.)

What’s the difference between gegen die Wand and an die Wand?

an die Wand: “to the wall,” neutral placement next to/at the wall; final position is by the wall (usually touching).
gegen die Wand: “against the wall,” emphasizes contact/pressure or leaning. With motion verbs like fahren/rennen, gegen can mean collision: Ich fahre gegen die Wand = I crash into the wall. For ordinary placement, an die Wand stellen is more common; gegen sounds like leaning or pressing the object against the wall.

Why use stellen and not legen or setzen?

stellen: put something in an upright/standing position. Result: it steht.
legen: lay something down horizontally. Result: it liegt.
setzen: seat someone/something (or reflexive: sich setzen = sit down). Result: it sitzt. A chair is normally upright, so stellen is idiomatic. You’d only use legen for a chair if you lay it on its side/back.

What’s the difference between stellen and stehen here?

stellen (action, transitive): placing something upright somewhere. Takes an object and typically a direction phrase: Ich stelle den Stuhl an/gegen die Wand.
stehen (state, intransitive): being in an upright position somewhere. Der Stuhl steht an der Wand. Think: action (Accusative, “where to?”) vs. location (Dative, “where?”) with two-way prepositions like an.

How do I say this in the past?

Conversational past (Perfekt): Ich habe den Stuhl gegen die Wand gestellt.
Simple past (Präteritum, more written): Ich stellte den Stuhl gegen die Wand.

Does gegen die Wand sound violent, like smashing?

Not by itself. With stellen, it usually implies placing so that it touches/leans. For violent action, Germans typically change the verb:

Ich werfe/schlage den Stuhl gegen die Wand (throw/hit the chair against the wall).
With fahren/rennen
- gegen, it does imply collision.

How do I pronounce the tricky parts?

Ich: [ɪç] (the “ich”-sound, not like English “k”).
Stuhl: initial St- is often [ʃt] in Standard German → [ʃtuːl].
stelle: [ˈʃtɛlə].
W in Wand: like English “v” → [vant].
gegen: [ˈgeːgən] (hard g, long e).
These are Standard German approximations; regional accents may vary.

Can I change the word order?

Yes, for focus or style:

Den Stuhl stelle ich gegen die Wand. (emphasis on “the chair”)
Gegen die Wand stelle ich den Stuhl. (emphasis on the location)
The finite verb (stelle) stays in second position in main clauses.

What are the genders, and how would the articles change if they were indefinite?

der Stuhl (masculine) → accusative: den Stuhl; indefinite: einen Stuhl.
die Wand (feminine) → accusative: die Wand; indefinite: eine Wand.

What’s the difference between Stuhl, Sessel, and Sitz?

Stuhl: a chair (no arms typically).
Sessel: an armchair (comfy, upholstered).
Sitz: a seat (the sitting place, not the furniture item itself).

Is stellen separable? Are there related verbs?

stellen itself is not separable. There are common separable derivatives:

hinstellen: to set/put down (there). Ich stelle den Stuhl hin.
abstellen: to set down/park/switch off.
aufstellen: to set up/erect.
umstellen: to rearrange; also “surround” (context-dependent).
verstellen (inseparable): to adjust or obstruct (change meaning).

How do I say the chair is leaning against the wall?

Action: Ich lehne den Stuhl an die Wand. / … gegen die Wand.
Result: Der Stuhl ist an die Wand gelehnt. or more commonly: Der Stuhl lehnt an der Wand.

When do I use dative with an?

With location (no movement):

Movement (accusative, wohin?): Ich stelle den Stuhl an die Wand.
Location (dative, wo?): Der Stuhl steht an der Wand.

Can I drop the article, like “Ich stelle Stuhl …”?

No. Countable singular nouns in German normally need an article. Use den/einen Stuhl. You can drop the article in the plural: Ich stelle Stühle an die Wand.

Why not zur Wand?

zu(r) means “to/toward” but not “up to and touching.” For placing something at a surface, German uses an (or here, gegen for contact/pressure). Ich stelle den Stuhl zur Wand is not idiomatic; say an die Wand or gegen die Wand instead.

Is there a difference between Wand, Mauer, and Wall?

Wand: a wall of a room/building (typically interior).
Mauer: a (usually outdoor) masonry wall (e.g., garden wall).
Wall: an embankment/rampart; not a normal house wall.
So in a room you’d almost always say Wand.