Learn Hardcore Turkish: Ev sahibi giriş katındaki posta kutusuna bir uyarı bıraktı. - The landlord left a warning in the mailbox on the ground floor.

a landlord/landlady renting out a place,
the homeowner,
the host (the person whose place you’re visiting). It’s gender‑neutral. An old-fashioned feminine form is ev sahibesi, but most people just use ev sahibi for everyone.

How is giriş katındaki formed and what does the -ki do?

It’s a stacked formation:

giriş = entrance
kat = floor/storey
giriş katı = the entrance/ground floor (compound; note the possessive -ı on kat)
giriş katında = on the entrance floor (locative)
giriş katındaki = the one that is on the entrance floor (locative + -ki turns it into an adjective meaning “the one at/in/on …”) So giriş katındaki posta kutusu = “the mailbox that is on the ground/entry floor.”

Could I say girişteki or giriş katta instead?

girişteki posta kutusu = “the mailbox at the entrance” (near/by the entry area). This focuses on the doorway/entrance space itself.
giriş katındaki posta kutusu = “the mailbox on the ground/entry floor.” This points to that floor level.
giriş katında is the natural locative form; giriş katta (without the possessive) is less idiomatic. If you want an adjective, use giriş katındaki.
All three can be understood; choose based on whether you mean the entrance area vs. the ground-floor level.

Is giriş katı the same as “ground floor”? How is numbering done in Turkey?

Yes. giriş katı or zemin kat both mean “ground floor.” In Turkey:

giriş/zemin kat = ground floor (street level)
birinci kat = the first floor above the ground floor So Turkish “birinci kat” aligns with British usage (first floor above ground), not American (where “first floor” is ground level).

Why does posta kutusu have that ending -su?

It’s an indefinite compound noun: posta (post) + kutu (box) → posta kutusu (mailbox). In Turkish, the second noun in such compounds takes a 3rd-person possessive marker (here -su) even though it’s not literal possession. Compare:

posta kutusu = mailbox (compound)
binanın posta kutusu = the building’s mailbox (true possession with genitive on the first noun)

Why is it posta kutusu-na and not posta kutusu-ya?

Because when you add the dative to a 3rd-person possessed form (like kutusu), Turkish uses the buffer -n-:

kutu → kutusu (its box / compound head)
Dative: kutusuna (“to the box”) So it’s -na/-ne after 3rd-person possessive, not -ya/-ye.

Why the dative case (posta kutusuna) and not the locative (posta kutusunda)?

With verbs of placing/putting like bırakmak (to leave) or koymak (to put), Turkish typically uses the dative to mark the goal/target:

posta kutusuna = to/into the mailbox If you want to be explicit about “into,” you can say:
posta kutusunun içine = into the mailbox Using the locative (posta kutusunda) here would describe a location rather than the goal and is not the natural choice with bırakmak.

Why is it bir uyarı and not uyarıyı?

Because the direct object is non-specific/indefinite. In Turkish:

Indefinite direct objects are unmarked for accusative: bir uyarı bıraktı = “(He/She) left a warning.”
The accusative (uyarıyı) marks a specific/known item: uyarıyı bıraktı = “(He/She) left the (particular) warning.” So here, the speaker isn’t pointing to a specific warning already known to both parties.

Do I have to include bir? What changes if I drop it?

You can drop it: … uyarı bıraktı is still grammatical and indefinite. Using bir is very common and often sounds more natural in this kind of sentence; it lightly emphasizes “a/some (one) warning.” Without bir, it can sound a bit more generic.

What exactly does uyarı mean? Are there other options?

uyarı = warning/notice (neutral, everyday). Alternatives:

ihtar = warning (more formal/legal)
not / uyarı notu = a note/a warning note
tebligat = formal written notification (bureaucratic/legal) Pick based on formality. For a sticky note or slip, uyarı notu fits well.

Why use bıraktı instead of koydu?

bırakmak = to leave (behind), often used for leaving notes/messages.
koymak = to put/place (neutral placing action). Both are possible here; bıraktı is very idiomatic for “left a (warning) note.” For physically placing something without the “left behind” nuance, koydu is fine.

Can I change the word order?

Yes, Turkish is flexible, though the neutral placement keeps the indefinite object near the verb. Examples with slight shifts in emphasis:

Giriş katındaki posta kutusuna ev sahibi bir uyarı bıraktı. (emphasizes the location)
Ev sahibi bir uyarı, giriş katındaki posta kutusuna bıraktı. (comma adds rhetorical emphasis) Keep the verb near the end; avoid moving an indefinite object too far from the verb unless you’re aiming for a marked/emphatic style.

Any pronunciation tips for tricky letters like ı and ş in this sentence?

ı (dotless i) in bıraktı, uyarı, katındaki: a close, central vowel, like the a in “sofa” or the e in “taken,” but shorter.
ş in giriş: “sh” as in “ship.”
Stress is usually on the last syllable: e.g., ev sa-Hİ-bi, gi-RİŞ, ka-TIN-da-Kİ, u-ya-RI, bı-rak-TI.

Could this sentence also mean “forgot a warning there”?

No. bırakmak implies intentional leaving/placing. “Forgot” would be unutmak:

Ev sahibi … bir uyarıyı orada unuttu. = “The landlord forgot a (particular) warning there.”
For posting/attaching a notice, you might use yapıştırmak (to stick) or asmak (to hang):
… posta kutusunun üzerine bir uyarı yapıştırdı/asdı.