Learn Hardcore German: An der alten Mauer wachsen bunte Blumen.

an is a two-way preposition. It takes the dative for location (answering Wo? “Where?”) and the accusative for direction (answering Wohin? “To where?”).
Here it describes a location at the wall, so it’s dative: an der alten Mauer.
Contrast: Ich hänge das Bild an die Wand (motion → accusative) vs. Das Bild hängt an der Wand (location → dative).

Why is it der alten Mauer and not die alte Mauer or dem alten Mauer?

Mauer is feminine: die Mauer.
Dative singular feminine is der, so with dative after an you get an der.
dem is dative for masculine/neuter, so it doesn’t match a feminine noun. die alte Mauer would be nominative/accusative, not dative.

Why does the adjective end in -en in der alten Mauer?

After a definite article (like der/die/das), adjectives take the “weak” ending.
In dative singular (any gender), that ending is -en, hence der alten Mauer.

Is An der alten Mauer the subject?

Why does the verb come before bunte Blumen?

German main clauses follow the “verb-second” (V2) rule.
A whole phrase (An der alten Mauer) is placed in the first position, so the finite verb (wachsen) must be second, and the subject (bunte Blumen) follows it.

Can I start with the subject instead?

Yes: Bunte Blumen wachsen an der alten Mauer.
Both orders are correct. The original puts emphasis on the location; starting with the subject emphasizes what grows.

Why is it wachsen and not wächst?

The verb agrees with the subject. Blumen is plural, so the verb is wachsen (3rd person plural).
Mini paradigm: ich wachse, du wächst, er/sie/es wächst, wir wachsen, ihr wachst, sie/Sie wachsen.

Why is there no article before bunte Blumen?

Indefinite plural nouns in German often drop the article, similar to English “(some) colorful flowers.”
If you want to be definite, use die: Die bunten Blumen wachsen … If you want to be fully indefinite singular, you’d use eine (e.g., eine Blume).

How do I negate this sentence?

Use kein- for negating an indefinite noun: An der alten Mauer wachsen keine bunten Blumen.
You can also front the negation: Keine bunten Blumen wachsen an der alten Mauer.
Don’t use plain nicht directly before the noun here; with indefinite nouns, kein- is the natural choice.

How do I turn it into a yes/no question?

Put the verb first: Wachsen an der alten Mauer bunte Blumen?
You can also put the subject right after the verb: Wachsen bunte Blumen an der alten Mauer?

How would it look in a subordinate clause?

The finite verb goes to the end: …, weil an der alten Mauer bunte Blumen wachsen. / …, dass an der alten Mauer bunte Blumen wachsen.

What’s the difference between an der Mauer, auf der Mauer, and bei der Mauer?

an der Mauer: at/against/by the wall (often implying contact or immediate proximity).
auf der Mauer: on top of the wall.
bei der Mauer: near the wall (in the vicinity, not necessarily touching).

Why Mauer and not Wand?

die Mauer typically means an exterior or freestanding wall (garden wall, city wall).
die Wand is usually an interior wall inside a building.
Flowers are described as growing by an outdoor wall, so Mauer fits best.

Can I contract an der to am here?

No. am = an dem (masculine/neuter dative). Mauer is feminine dative (der), so you must say an der.
Example of a valid contraction: am alten Haus (= an dem alten Haus).

Does an ever take the accusative in similar sentences?

Yes, with motion toward the wall: Er lehnt die Leiter an die Mauer.
With growth, German typically expresses “along/at the wall” with dative: Efeu wächst an der Mauer (hoch). Saying an die Mauer wachsen is unusual for flowers.

How do you pronounce wachsen?

w sounds like English “v.”
chs is pronounced like “ks” (as in “foxy”): roughly “VAHK-sen.”
Don’t pronounce ch separately here; the sequence chs simplifies to that “ks” sound.

Can bunt come after the noun instead?

As a predicate adjective, yes: Die Blumen sind bunt.
When it directly modifies a noun (attributive), it goes before the noun and takes endings: bunte Blumen, keine bunten Blumen, der bunten Blumen (depending on case/article).