Learn Hardcore Turkish: Unutkan arkadaşım üyelik formunu iki kez doldurdu. - My forgetful friend filled out the membership form twice.

Unutkan is an adjective meaning forgetful. In Turkish, adjectives come before the noun they modify.
arkadaş = friend; -ım = my (1st person singular possessive). So arkadaşım = my friend.
Together: Unutkan arkadaşım = my forgetful friend. No article is needed in Turkish.

Why is it üyelik formunu and not just üyelik formu?

Because the direct object is specific/definite, Turkish marks it with the accusative case. The base compound is üyelik formu (membership form). To say “the membership form” as a definite object, add accusative after the compound suffix:

form-u (3sg possessive from the compound) + buffer -n-
- accusative -u → formunu. So: üyelik formunu “the membership form” (as a definite object).

How does the compound üyelik formu work? Why does the head noun have a possessive suffix if no one owns it?

This is an “indefinite noun compound” (belirtisiz isim tamlaması): N1 + N2-3sg.POSS, meaning “N2 of/for N1” as a type.

üyelik (membership) + form-u (form-3sg.POSS) → “membership form.”
The -u here is part of the compound pattern; it doesn’t imply a real possessor.
Other examples: okul kapısı (school door), çay bardağı (tea glass).

Does formunu mean “his/her form”? How do I tell it’s not possessed by a person?

It can mean that if you add a genitive possessor: onun formunu = his/her form (accusative). In this sentence, üyelik formunu is a compound (“the membership form”), not someone’s personal form. Context (and presence/absence of a genitive like onun) disambiguates.

What’s the difference between üyelik formunu and bir üyelik formu?

üyelik formunu (accusative) = a specific, known membership form (“the membership form”).
bir üyelik formu (no accusative) = an unspecified membership form (“a membership form”).
You can also omit bir and say üyelik formu doldurdu for an indefinite object in general contexts; adding bir makes the “one/a” meaning explicit.

What does iki kez mean? Can I also say iki kere or iki defa?

All three mean “twice.” They’re interchangeable in most contexts.

kez: neutral/standard (a bit more formal in tone).
kere: very common/colloquial.
defa: also common (from Arabic); neutral.

Where can I place iki kez in the sentence?

Both are natural:

Unutkan arkadaşım iki kez üyelik formunu doldurdu.
Unutkan arkadaşım üyelik formunu iki kez doldurdu. Adverbs of frequency typically appear before the verb. Placing it after the verb (… doldurdu iki kez) is unusual in standard prose.

What tense/person is doldurdu? Why is there no subject pronoun?

doldur- (to fill in/out) + past -DI → doldurdu = “he/she filled out.”
3rd person singular in the simple past has no extra personal ending.
Turkish usually omits subject pronouns when the subject is clear (here it’s Unutkan arkadaşım).

Why doesn’t the verb agree with the -ım (“my”) on arkadaşım?

-ım marks possession on the noun (my friend). The subject itself is still third person (that friend), so the verb is 3sg: doldurdu, not a 1st-person form. Compare: Annem geldi (My mother came) → verb is 3sg.

Do I need to say Benim unutkan arkadaşım?

Not usually. The possessive suffix (-ım) already expresses “my.” Adding benim is optional and is used for emphasis, contrast, or clarity (e.g., when multiple possessors are in play).

What’s the difference between Unutkan arkadaşım and Arkadaşım unutkan?

Unutkan arkadaşım: attributive; you’re identifying a specific friend by this quality (“my forgetful friend”).
Arkadaşım unutkan: predicative; you’re stating a fact about your friend (“my friend is forgetful”).
In careful/formal style, you might see Arkadaşım unutkandır; the -dır copula is often optional in speech.

Any vowel harmony or buffer-letter rules illustrated here?

arkadaş-ım: the possessive -Im appears as -ım because the last vowel in the stem is back-unrounded (a).
form-u: 3sg possessive -u because the last vowel in the stem is back-rounded (o).
form-u-n-u: when adding a case after a possessive suffix, insert buffer -n-.
doldur-du: past -DI surfaces as -du by harmony with the preceding back-rounded vowel.
For vowel-final stems without a possessive, buffer -y- is used before case: araba-y-ı.

How do I make it plural: “My forgetful friends filled out the membership form twice”?

Unutkan arkadaşlarım üyelik formunu iki kez doldurdu(lar).
With human plural subjects, the verb’s plural marker -lar is optional; both forms are correct.