Consonant Mutation
In Turkish, there are certain consonants that are replaced by other letters when suffixes are added to them.

When we add a suffix to word that ends in "p/ç/t/k" and the consonant has a vowel both before and after it, the consonant will change:

p ➨ b
ç ➨ c
t ➨ d
k ➨ ğ/g
Examples:

(kitap) Benim kitabım - My book
(dolap) Dolaba koydum - I put it into the cupboard

(ağaç) Ağaca bak - Look at the tree
(borç) Borcum ne kadar? - How much do I owe?

(yoğurt) Hakan yoğurdu yedi - Hakan ate the yogurt
(dört) Saat dörde çeyrek var - The time is a quarter to four

(sözlük) İngilizce-Türkçe sözlüğün var mı? - Do you have an English-Turkish dictionary?
(çocuk) Fatih çocuğa şeker verdi - Fatih gave candy to the child
The letter "k" is replaced by a "g" instead of a "ğ" when it has a "n" before it and a vowel after it:

(renk) Göz rengi - Eye color
(kepenk) Ömer kepengi kapattı - Ömer closed the shutter

EXCEPTIONS
In general, single syllable words don't have consonant mutation applied to them:

(top) Benim topum - My ball
(koç) Çobanın koçu - The shepherd's goat
(süt) Sütü bardağa dök - Pour the milk into the glass

Words that are derived from other languages (French, English, Arabic, etc.) don't experience any consonant mutation:

(bilet) Biletini unutma - Don't forget your ticket
(internet) İnterneti her gün kullanıyorum - I use the internet everyday
(hukuk) İslam hukuku - Islamic law

Proper nouns such as names of people or places also do not change:

(Zeynep) Zeynep'e kalem verdim - I gave Zeynep a pen
(Haliç) Haliç'e gittik - We went to the Golden Horn
(Tokat) Tokat'ın nüfusu - The population of Tokat