Merhaba herkese!

I had a question about a subject that has been confusing me a lot lately

What´s the difference between when I say:

Kafasını kırarım

or

Kafasıyı kırarım

when do I use the "yı" or "nı"?

I know the sentences are a bit weird hahaha XD that´s all I could think of though,

Thanks!

The buffer -n is used when the noun has already been suffixed.  

See here: http://www.turkishlanguage.co.uk/buffer02.htm 

Kedisi his/her cat becomes kedisini [kedi-si-ni] his/her cat [3rd person possessed object]

Yes but also:

o = s/he,it,that

o-n-u = him,her,it,that (accusative case)

bu = this

bu-n-u = this (accusative case)

şu = this

şu-n-u = this (accusative case)

etc.

Yes but the question was "when do I use the "yı" or "nı"?" 

But you are right I adjusted a little.

The rule may be stated like this:

The case suffixes are preceeded by -n- when a pronoun/possessive suffix is involved.

o -> o-n-u, o-n-a, o-n-da, o-n-dan, o-n-un-la

if a stem ends with consonant (ex: ev):

ev-i  -> ev-i-n-i, ev-i-n-e, ev-i-n-de, ev-i-n-den (-i is possessive suffix in ev-i)

ev -> ev-i, ev-e, ev-de, ev-den (no possessive suffix here)

and if a stem ends with a vowel (ex: anne):

anne-si -> anne-si-n-i, anne-si-n-e, anne-si-n-de, anne-si-n-den (-si is possessive suffix in anne-si)

anne -> anne-y-i, anne-y-e, anne-de, anne-den (-y- only appears when a suffix starts with a vowel)

There are also two words that are exceptions when they take possesive suffix ;

Su [water]  and Ne [what] appear in different form when they take possesive suffix ;

Su + y + um = Suyum [My water ] [ instead of "sum" ]

Su + y + un = Suyun [Your water ] [ instead of "sun" ]

Su + y + u =  Suyu [ His/Her/its water] [ instead of "susu" ]

etc.....

--------------------------------------------------------------------------

Ne [what] + y + im --> Neyim [ what ......of me]

Ne [what] + y + in ---> Neyin [ what ......of you ]

Ne [what] + [y] + [s]i ---> Neyi [ what ......of he/she]

etc...

--------------------------------------------------------------------------

Note : * For the word "Ne" the normal forms [ Nem, Nen, Nesi,Nemiz,Neniz,Neleri] are also acceptable however the exceptional forms [Neyim, Neyin, Neyi , Neyimiz,Neyiniz,Neleri] are used commonly.