I am a little confused about something.

masa= table

masayi=the table...is this correct

masayi kose´de=the table is in the corner  veya masa kose´de

another example

piyano=piano

piyanoyu = the piano

piyanoyu kose´de = the piano is in the corner....veya piyano kose´de

It´s a good question.

masayı=the table...is this correct?

It´s neither correct nor incorrect. Turkish doesn´t have articles (i.e. a, the) unlike English, French, Spanish etc... So my advice is to not think in this way.

Masayı sattım. This would be the typical case in Tukish. The verb satmak requires (y)i. True, it does translate into: I sold the table.

Bir masa sattım. This is similar to ´a´ in English, we don´t add the y(i). I sold a table.

Masamı sattım. We still need the (y)i here. We wouldn´t say in English: I sold the my table.

Also, there is no apostrophe: köşede

thanks Denizli