Learn Hardcore Turkish: Kız kardeşim yeni kulübe üye olurken kısa bir form doldurdu. - While my sister was joining the new club, she filled out a short form.

Kız kardeşim yeni kulübe üye olurken...
My sister (normal, neutral).
Benim kız kardeşim yeni kulübe üye olurken...
My sister (as opposed to someone else’s), with emphasis.

So the sentence is perfectly natural without benim.

What exactly does the -im in kız kardeşim do?

The suffix -im is the 1st person singular possessive ending:

ev = house
evim = my house
annem (anne + m) = my mother
kız kardeşim (kız kardeş + im) = my sister

So -im attaches to the possessed noun and means “my”. Turkish normally does not use a separate word like my unless there is emphasis.

Why is it kulübe and not kulüp in yeni kulübe?

Two things are happening:

Case ending (-e)
Turkish marks direction / “to” with the dative case:
- kulüp = club
- kulüp + -e → kulübe = to the club
Consonant softening (p → b)
In many Turkish words, final p, t, k, ç soften to b, d, ğ, c when a vowel suffix is added:
- kitap → kitaba (to the book)
- renk → renge (to the color)
- kulüp → kulübe (to the club)

So kulübe literally means “to the club” and yeni kulübe = “to the new club”.

What is the role of yeni in yeni kulübe? Why does it come before the noun and not after it like in English?

Yeni is an adjective meaning “new”.

In Turkish, adjectives always come before the noun they describe and never change form:

yeni kulüp = new club
yeni kulübe = to the new club
yeni ev = new house
yeni evi = the new house (object)

Unlike some languages, Turkish adjectives do not agree in number or case. The case ending goes on the noun only:

yeni kulüp (no case)
yeni kulübe (dative case, -e on kulüp)

What does üye olmak mean, and why is it two words?

Üye olmak literally means “to be/become a member”, and in many contexts it corresponds to English “to join (a club, organization, etc.)”.

Turkish often uses light verb constructions:

üye olmak = to become/be a member
karar vermek = to decide (literally “to give a decision”)
yardım etmek = to help (literally “to make/give help”)

Here üye (member) is a noun, and olmak (to be / to become) is the verb. Together they function like a single verb: “to join / to become a member (of)”.

Why is it üye olurken and not something like üye olduğunda or üye olunca?

The ending -ken means “while / when (doing something)”, showing an action happening at the same time as another.

olmak (to be/become)
olur (aorist: becomes / is (in general))
olurken = while (she/he) is becoming / joining

So üye olurken means “while (she) was joining / becoming a member”, and it sets the time for the main action (doldurdu).

Alternatives:

üye olduğunda – when she became a member (more like “at the time when”)
üye olunca – when she became a member / once she became a member

-ken feels more like two actions happening at the same time:

While she was joining the new club, she filled out a short form.

What does kısa bir form show about definiteness? Why is bir used here, and can it be omitted?

Bir is the basic word for “one”, but before a noun it often works like the English indefinite article “a/an”.

kısa form = short form
kısa bir form = a short form

In most contexts like this, bir is either:

a neutral “a/an”, or
slightly more specific than no bir at all.

You can say:

Kız kardeşim ... kısa bir form doldurdu.
Kız kardeşim ... kısa form doldurdu.

Both are grammatically correct. Without bir, it often sounds a bit more generic or neutral. With bir, it feels more like a particular form she had to fill in (but not a known, specific one).

Why is it form and not formu in kısa bir form doldurdu?

This is about definiteness and specificity of the object.

In Turkish:

A specific/definite direct object usually takes -ı / -i / -u / -ü (accusative).
A non-specific/indefinite direct object usually stays in the bare form (no ending).

Compare:

Kısa bir form doldurdu.
She filled out a short form (some form, not specific to the listener).
Kısa formu doldurdu.
She filled out the short form (a particular, known form).

In the given sentence, the idea is just that when she joined the club, she had to fill a short form, not that the speaker is referring to a particular, previously mentioned form. So the bare form (with no -u) is natural.

What tense is doldurdu, and what does it tell us about the action?

Doldurdu is the simple past tense of doldurmak (to fill in/to fill out).

doldur- = verb stem
-du = simple past ending (here with u by vowel harmony)

It indicates:

The action is completed.
The speaker is sure it happened (not hearsay).

Compare:

doldurdu = she filled it out (finished action, factual)
dolduruyordu = she was filling it out (past continuous, focuses on the ongoing process)

In the sentence, doldurdu presents the form-filling as a completed event that happened while she was joining the club.

Is the word order fixed? Could we move üye olurken or kısa bir form around?

Turkish word order is flexible, but the default is Subject – (time/place/manner) – Object – Verb, with the main verb at the end.

Current order:

Kız kardeşim (subject)
yeni kulübe üye olurken (time clause: when/while joining the new club)
kısa bir form (object)
doldurdu (verb)

You could also say:

Kız kardeşim kısa bir form doldurdu, yeni kulübe üye olurken.
Yeni kulübe üye olurken kız kardeşim kısa bir form doldurdu.

All are grammatical, but changing the order can slightly change what is emphasized. The verb must stay at or very near the end in standard style.

Are there any important sound changes or harmony rules to notice in this sentence?

Yes, a couple:

Vowel harmony in suffixes
Suffix vowels usually adjust to the last vowel of the stem:
- kulüp → kulüp + e → kulübe (front, unrounded vowel: e)
- doldur- + du → doldurdu (last vowel u, so suffix uses u)
Consonant softening (p → b)
As mentioned, kulüp ends in p, which often softens to b when a vowel suffix is added:
- kulüp + e → kulübe

These are typical Turkish phonological processes that you’ll see very often when you add case endings or tense endings to nouns and verbs.