Learn Hardcore Turkish: Kız arkadaşım geç kaldığı için benden özür diledi, ben de gülümsedim. - My girlfriend apologized for being late, and I smiled.

It’s the 1st person singular possessive suffix (-ım = my). In the compound kız arkadaş (girl + friend → girlfriend), the head is arkadaş, so the possessive attaches to the head: kız arkadaş-ım = my girlfriend. You don’t put a possessive on kız or on both words.

Does kız arkadaşım always mean “my girlfriend,” or can it mean “my female friend”?

In modern usage it usually means “my girlfriend” (romantic). To avoid ambiguity:

For a romantic partner: sevgilim is crystal clear; kız arkadaşım also commonly means this.
For a female friend: rely on context, say bir kız arkadaşım, or just arkadaşım (Ayşe). Some say kadın arkadaşım in formal settings, but it’s less common.

Why is it benden after özür diledi? Why the -den ending?

Because the verb phrase özür dilemek (“to apologize”) governs the ablative: you apologize “from” someone in Turkish.

Pattern: birinden özür dilemek = to apologize to someone.
Pronouns: benden, senden, ondan, bizden, sizden, onlardan.

Can I say bana özür diledi?

No. That’s unidiomatic. Use the ablative: benden özür diledi. The dative (bana) is wrong with özür dilemek.

What is going on morphologically in geç kaldığı?

geç kal- = to be late (adverb geç
- verb kal-).
-DIK nominalizer forms a verbal noun/relative clause.
Add 3sg possessive (-ı/ -i/ -u/ -ü) to link the (understood) subject of the clause. Putting it together: kal-dık-ı → kal-dığ-ı (the k softens to ğ before a vowel). So geç kal-dığ-ı ≈ “his/her having been late.” The ğ doesn’t make a hard sound; it lengthens the preceding vowel slightly.

Why not just say geç kaldı için?

Because için (“because/for”) takes a noun phrase. kaldı is a finite verb form. You must nominalize the verb: geç kaldığı için (“because of her being late” → “because she was late”).

Could I use çünkü instead of için?

Yes, but you must restructure into two clauses:

Kız arkadaşım benden özür diledi, çünkü geç kaldı.
You can also use past perfect nuance: çünkü geç kalmıştı. Don’t combine çünkü and için redundantly in the same reason.

Who is understood to be late in geç kaldığı için?

By default, it refers to the subject of the main clause (kız arkadaşım). If someone else was late, mark that subject in genitive:

Kız arkadaşım, kardeşinin geç kaldığı için benden özür diledi. (“My girlfriend apologized to me because her sibling was late.”) Another option: X’in geç kalması yüzünden/ nedeniyle.

What exactly does ben de mean here?

It means “I too/I also,” and in context often reads like “so I (also) …” It links your action to the previous one: she apologized, and I (also) smiled in response.

Is de here the same as the locative -de/-da? Why is it written separately?

No. This de/da is the additive clitic “also/too,” written as a separate word. It follows vowel harmony (de/da) but never becomes te/ta. Compare:

Ben de gülümsedim. = I also smiled. (clitic, separate)
Bende para var. = I have money (on me). (locative suffix, attached to the noun)

Do I need the pronoun in ben de gülümsedim?

Not strictly. Gülümsedim already encodes “I.” Adding ben gives contrast/emphasis (“I, too”), which fits the discourse here.

Can I move parts around? For example, start with the reason or move benden?

Yes, Turkish word order is flexible for information structure:

Geç kaldığı için kız arkadaşım benden özür diledi, ben de gülümsedim.
Kız arkadaşım, geç kaldığı için benden özür diledi.
Kız arkadaşım benden özür diledi (geç kaldığı için). You can also say: Benden özür diledi or özür diledi benden (the latter is more colloquial/emphatic).

What tense/ending is -di in diledi and gülümsedim?

It’s the simple past (-di/-dı/-du/-dü with vowel harmony).

dile-di = (s/he) apologized.
gülümse-di-m = I smiled. The -miş past (-mişti, etc.) would add reported/evidential nuance.

Are there alternatives to … için for “because of being late”?

Yes:

geç kaldığından (dolayı) = because (he/she) was late.
geç kalması yüzünden/ nedeniyle = due to his/her being late.
Colloquial: geç kaldı diye.

Pronunciation tips for kaldığı?

ğ is a “soft g”: it lengthens/smooths the preceding vowel, not a hard consonant.
ı (dotless i) is a high back unrounded vowel, like a relaxed “uh” sound. So roughly: kal-dı-ɣı (with a lengthened ı before ğ).