I've just found a water bottle that we brought back from Türkiye - we used it to carry gin and tonic to drink on the journey back but that's another story

On the bottle it says doğal kaynak suyu which I translate as "natural spring water". But why is water written in the accusative noun state and not the nominal state???

Why not doğal kaynak su?

Kaynak suyu is a noun modification. In Turkish, when a noun is modified by, specified by another noun, the modified one, that is the second noun, takes a suffix. If the modified noun ends with a consonant, the suffixes are "-i,-ı,-ü,-u": Tren istasyonu, Toplantı salonu, Kadın kuaförü, etc.

If the modified noun ends with a vowel the suffixes are "-si,-sı,-sü,-su": Polis arabası, Kadın dergisi, Çamaşır makinası.

When "su" is modified noun, it breaks the rule for the vowel ending modified noun and takes "y" buffer. "Su", as a modified noun, is an exception.

Kaynak suyu
Portakal suyu
Meyve suyu
Çamaşır suyu
Tavuk suyu

I read that's because "su" is a word with only one syllable, hence the -y- as a fusion consonant. Is that true then for all one-syllable words ending in a vowel? I'd guess so?

Elisa

It is not the accusative case what you are asking. It is the third singular possessive form. "suyu" as in "kaynak suyu" has no case suffix.

There are two different things.

1. "Suyu" as accusative (direct object)

The word "su" is no exception with the accusative case.
The accusative case takes -ı, -i, -u ,-ü, . When after a vowel we add a buffer letter "y".

Kalem+i ver. > Give me THE pen. (the pen is the direct object).
Okul+u sevmiyorum. > I don't like THE school. (School is the direct object.)
Araba+yı getir. > Bring the car.
Su+yu iç. > Drink THE water. (Again the water is the direct object.)
Konu+yu değiştir. > Change the subject.

As you see with the accusative suyu is regular as any other word.

2. "Suyu" as third possessive form of su:
Lets start with other examples:

kalem: pen
Benim kalem+im. > My pen.
Senin kalem+in. > Your pen.
Onun kalem+i. > His/Her/It's pen.
Bizim kalem+imiz. > Our pen.
Sizin kalem+iniz. > Your pen.
Onların kalem+i. (or kalemleri) > Their pen.

kedi : cat
Benim kedi+m. > My cat.
Senin kedi+n. > Your cat.
Onun kedi+si. > His/Her/It's cat.
Bizim kedi+miz. > Our cat.
Sizin kedi+niz. > Your cat.
Onların kedi+si or (kedileri) > Their cat.


su: water
Benim su+yum. > My water.
Senin su+yun. > Your water.
Onun su+yu. > His/Her/It's water.
Bizim su+yumuz. > Our water.
Sizin su+yunuz. > Your water.
Onların su+yu. (or suları > Their water.

Suyu is an exception for possessive suffixes not for accusative case.

Here you can practice possessive suffixes:
http://users.pandora.be/orientaal/Tu01b.html

The following information has been copied from that page:
http://www.turkishlanguage.co.uk/possadj.htm

Suffix Added to Words ending in Consonants

-ım -im -um -üm - my
-ın -in -un -ün - your
-ı -i -u -ü - his/her/its
-ımız -imiz -umuz -ümüz - our
-ınız -iniz -unuz -ünüz - your
-ları -leri - their

Suffix added to Words ending in Vowels

-m
-n
-sı -si -su- -sü
-mız- miz -muz -müz
-nız -niz -nuz -nüz
-ları -leri