This is how I understand it (I´m a learner as well):
In English when you use one noun to describe another noun, it´s pretty simple, you pretty much juxtapose the 2 nouns, for example: telephone number, orange juice, weather report, etc.
In Turkish, when you do that, you need to use what is called "izafet" construction, which is the first noun in nominative case (no ending) and the second noun in accusative case (with -i ending), so for example:
That´s not accusative, it´s possesive suffix for 3rd sing. person (=her/his/its).
"telephone number" becomes "telefon numara-s-ý" (s is the buffer letter)
If you add accusative suffix it would be "telefon numara-sý-ný"
It may sound weird but when you say his/her telephone number it´s still the same
his telephone number = (onun) telefon numarasý
Elif´s telephone number = Elif´in telefon numarasý
Test it with 1st sing person for example
My telephone number = (benim) telefon numara-m (-sý is gone)
and it would be "telefon numara-m-ý" when accusative suffix is added.
It is confusing for learners because they are similar in many cases.
"orange juice" becomes "portokal su-y-u" (y is the buffer letter)
Actually y is not buffer letter it is because it was "suw" in old times
suy-u < suw-u
and it has become su (<suy<suw) when there is no suffix after it
"weather report" becomes "hava rapor-u"
Now Türk is a noun that is being used as an adjective (which is different from English, in English Turkish is an adjective, however in Turkish - Türk is a noun), so the 2nd noun it describes needs to be in accusative case, so therefore you have:
Türk lirasý - Turkish lira
Türk halýsý - Turkish carpet
Türk çocuklarý - Turkish children
That´s how I understand it, hopefully others can expand on it if needed.
Add:
I was checking the lessons on this site and found this one that has some additional explanations about it (under noun clauses):
http://www.turkishclass.com/turkish_lesson_68