"Senayakkabıgiyersin."

I can't translate it, but I can try to explain it.

When two words together form a meaning of their own, you have a compound word (birleşik sözcük) - in English, these can be an "open compound" (two words written separately, e.g. "school bus"), a "hyphenated compound" (two words written with a hyphen between them, e.g. "get-together"), or a "closed compound" (two words written together, e.g. "airport").

Which of these spellings is used is not always fixed (sometimes two or even all three spellings are possible), and even when there is a clear standard, this can change over time, especially when the compound word starts to feel like a single word instead of one that is made up of others. For example, what used to be "e-mail" is now often "email".

In Turkish, there seems to be something similar: some compounds are written as two words (e.g. sırt çantası) and some as one word (e.g. ayakkabı); sometimes, both spellings seem to be possible (e.g. hava limanı, havalimanı).

Here, too, I imagine that in some cases, these may be felt as a single new word after a while, not a compound word made up of two individual words anymore.

I presume that sırt çantası always has the accusative sırt çantasını, and "my backpack" is sırt çantam -- the -sı ending is felt to be the possessive ending that characterises such compounds in Turkish.

But with ayakkabı then it seems that it is no longer felt to be ayak kabı "foot container", and so you have not ayakkapları for the plural (like sırt çantaları) or onun ayakkabı for possessive (like onun sırt çantası) or ayakkabını for the accusative (like sırt çantasını) -- but it seems to have moved all the way along this "open compound -- closed compound -- single word" route and so you have plural ayakkabılar, possessive onun ayakkabısı, and accusative ayakkabıyı.

An English equivalent might be those compound words that are formed from noun + adjective (like in French), as in "court-martial", which traditionally had the plural "courts-martial" (treating it as two words and pluralising only the noun portion) but now can also have the plural "court-martials" (treating it as a single hyphenated word which adds the plural marker only at the end). On the other hand, "father-in-law" can only have the plural "fathers-in-law", I think, not "father-in-laws", but perhaps that will also change one day.

Thank you very much for that detailed explanation. We call the situation of being as a single word "anlam kayması or kaynaşması" in Turkish. (Is my last sentence correct?) There are lots of rules about spelling of compound words. If you want you can look at those links:

http://tdk.gov.tr/index.php?option=com_content&view=article&id=221:Ayri-Yazilan-Birlesik-Kelimeler&catid=50:yazm-kurallar&Itemid=132

http://www.imla.dilimiz.com/TDK/a_bitisikyazilanbirlesikk.HTM

If you have any questions about that please feel free to ask me.