Learn Hardcore Spanish (Spain): Cuidado, ese callejón es resbaladizo porque los charcos no se secan. - Careful, that alley is slippery because the puddles don’t dry.

este = “this,” near the speaker
ese = “that,” at some distance (often near the listener or visible but not close)
aquel = “that (over there),” farther away or removed from the immediate context

Here ese suggests the alley is not right next to the speaker but not especially far either. You could say ese de ahí to emphasize “that one there.”

Is callejón masculine or feminine, and what’s its plural?

Callejón is masculine: el callejón. Plural: los callejones.
Note:

calle (street) is feminine: la calle
The suffix -ón often forms nouns; in callejón it ended up meaning a narrow alley. The plural loses the accent: callejones.

Why use ser (es resbaladizo) instead of estar?

ser
- adjective describes a general/typical characteristic: Ese callejón es resbaladizo = “That alley is (in general) slippery.”
estar
- adjective describes a temporary state: Ese callejón está resbaladizo = “That alley is slippery (right now).” If it’s always damp, es fits; if it’s just after rain, está is better.

What exactly does resbaladizo mean? Any synonyms or regional variants?

Resbaladizo = “slippery.” Agreement: resbaladiza (fem.), resbaladizos/as (pl.).

Common in Spain: resbaladizo
Latin America alternative: resbaloso
Avoid escurridizo for surfaces (it means “slippery/elusive” about people/objects, not floors). Signs in Spain typically say Suelo resbaladizo.

How do I pronounce the tricky parts (especially callejón and the j)?

callejón: stress on the last syllable (cal-ye-JÓN); j is a harsh (like the ch in Scottish “loch”).
ll is usually pronounced like English “y” in most of Spain (yeísmo): cal-ye-jón.
charcos: ch = [tʃ] as in “church.”
se secan: in most of Spain, c before a, o, u = [k]; here “secan” has [k].
cuidado: soft/voiced d between vowels [ð], like “th” in “this.”

Why is it porque (one word, no accent) and not por qué, porqué, or por que?

porque = “because” (conjunction of cause) → correct here.
por qué = “why” (interrogative/exclamative): ¿Por qué no se secan?
porqué = “the reason” (noun): el porqué de algo.
por que = “for which/that,” a rare sequence that appears with certain verbs/phrases.

What does the se do in no se secan?

It’s the pronominal/middle use of secar(se): “to dry (on its own).”

Transitive: El sol seca los charcos = The sun dries the puddles.
Pronominal/middle: Los charcos se secan = The puddles dry (by themselves).
So no se secan = “(they) don’t dry.” It’s not an impersonal “they don’t dry them.”

Could I say porque no se secan los charcos instead of porque los charcos no se secan?

Yes, both are correct. Spanish often places known/less-informative subjects after the verb, especially in negative statements.

porque los charcos no se secan (neutral)
porque no se secan los charcos (slightly more focus on the verb phrase “do not dry”)

Why plural los charcos? Could it be singular?

Plural fits when there are several puddles or you’re speaking generally about puddles in that alley. Singular is fine if you mean a specific puddle: …porque el charco no se seca.
You can also say: …porque hay charcos que no se secan (there are puddles that don’t dry).

Is the comma placement right?

Yes:

Comma after the interjection: Cuidado, …
No comma before porque in a simple causal clause: …es resbaladizo porque…

Any natural alternatives to the whole sentence?

Cuidado con ese callejón; está resbaladizo porque los charcos no se secan.
Ojo, ese callejón resbala; los charcos no se secan.
Mucho cuidado: ese callejón es muy resbaladizo.
Ese callejón es resbaladizo por los charcos que no se secan.

What’s the difference between callejón, calleja, pasaje, and pasadizo?

callejón: narrow alley (common, neutral term)
calleja: very narrow/little street; a bit literary/regional in Spain
pasaje: passage/covered walkway or small street (often named in cities)
pasadizo: passageway, sometimes covered or hidden

Does resbaladizo have to agree with callejón?

Yes. Adjectives agree in gender/number:

el callejón es resbaladizo
la calle está resbaladiza
los callejones son resbaladizos

Why the present tense no se secan? Could I use other tenses?

The present indicates a general/habitual fact. Other options:

no se están secando = “aren’t drying (right now)”
no se han secado = “haven’t dried (yet)”
Choose based on whether you mean a habit, a current process, or a recent result.

Any spelling pitfalls here?

resbaladizo with b (from resbalar), not “resvaladizo.”
callejón needs the accent on -jón.
ese (demonstrative) has no accent under current rules (avoid old ése).
porque (because) is one word without an accent.