I realised that almost every word that finish with ak/ek etc have another way to say it but there is no difference between them so how it´s possible?

I put just few words as example but there is many others,is there any rules for it or am i wrong?

balık/balığı

bacak/bacağı

ayak/ayığı

parmak/parmağı

böcek/böceğı

köpek/köpeği

please answer,i am confused. thanks 

this seen on words ending one of these four letters: p, ç, t, k

if afterwards comes a vowel they soften to similar pronounced letters, which are: b, c, d, ğ (rarely g)

kitap kitabı

ağaç ağacı

ümit ümîdi

petek peteği

p-b
ç-c
t-d
k-ğ (g)

single syllable words dont change.
çek çeke

proper nouns change but not written in order to protect proper noun.
Irak Irak´a (pronounced as Irağa but not written.)
Mehmet´e (mehmede)

I didn´t understood well,can you just tell me is there any difference between those words or they are same.If you know to explain it better i would be grateful my english is bad and google translator not helps.

balık/balığı 

bacak/bacağı

ayak/ayığı

parmak/parmağı

böcek/böceğı

köpek/köpeği

yes it´s the same word

as you know in turkish language nouns can take cases 

balık/balığı balığı  accusative case 

also they can take possessive endings 

onun balığı  

so this word will change his meaning depending from cases, possessive endings, etc 

it depends of the context 

it depends in what context you intend to use the word 

i wish i could explain better

Okay thanks,i understand better and better but still not so sure

Can you please tell me which way is correct in following sentences or they both are

Ben balığı yiyorum/Ben balık yiyorum

Sen balık yiyorsun/sen balığı yiyorsun

o balığı yiyor/O balık yiyor

Biz balığı yiyoruz/Biz balık yiyoruz

Siz balığı yiyorsunuz/Siz balık yiyorsunuz

onlar balığı yiyorlar/onlar balık yiyorlar

And do you know any link where i can read more about it?

http://www.turkishclass.com/turkish_lesson_66

http://www.turkishclass.com/turkish_lesson_29

maybe this can help you 

when you try to look up in a dictionary, use the original form and drop some suffixes.
they are exactly the same word, the same meaning. This process only makes it easy to pronounce.

meaning changes but not because of this letter change, because of suffixes. and this is another grammar topic.

çiçeği, drop i suffix and go back to original form: çiçek. now you can look it up in a dictionary.

Biz balığı yiyoruz/Biz balık yiyoruz

Lana,

Both these sentences are correct, but are slightly different in meaning.

The speaker using the first sentence is talking about a particular fish; the second sentence is about eating any fish.

Very much like,

We are eating the fish/We are eating fish.

teşekkürler Alpha