Nouns ending in ç, p, t or k can change their last consonant when followed by a vowel. Their last consonant changes into c, b, d and ğ respectively for each of these cases. As a general rule, nouns consisting of multiple syllables change their last consonant, and nouns consisting of single syllables don't.

Below you can see how ağaç, kitap, yoğurt and çocuk change when they are followed by a vowel.

We are walking to the tree.	(Biz) ağaca yürüyoruz.
I am reading the book.	(Ben) kitabı okuyorum.
Eat the yogurt.	Yoğurdu ye.
He is looking after the child.	(O) çocuğa bakıyor.

When the k is preceded by an n, as in the word renk which means color, the k is replaced with a g instead of a ğ when it is followed by a vowel.

Look at the color.

Renge bak.

Since these changes are irregular, you will have to remember for every noun that ends in a ç, p, t or k whether this letter changes or not when a vowel follows it.

Breakdown of Ağaca yürüyoruz.