Besides the vowel harmony rules, there are other basic rules that affect the way suffixes are used. A vowel following another is never allowed in Turkish, and there are rules to avoid these situations when they occur as a result of other rules. There are also rules about consonant harmony, that make some consonants change in certain cases.

2. Consonant Harmony

Vowel harmony rules cause the vowels of suffixes to be modified when they are added to some words. There are similar rules about consonants. However, you may feel that all these rules are too many just for a simple start. Then, I advice you to omit the consonant harmony rules when you want to say or write something, just for the beginning. You will still be understood. Consonant harmony is mainly for making speech more fluent, it does not have a major effect on understandability. You will eventually learn these if you decide to continue learning Turkish, as you read sentences or listen to Turkish speakers.

There are two different cases of consonant harmony - either the last consonant of the main word changes, or the first consonant of the suffix changes. The trouble making consonants in this case are p, ç, t and k. Let´s call the words that end with one of p, ç, t or k the trouble words. 

CASE A - Word mutation.

Two conditions must be satisfied for word mutation to occur:    

1. You have a word ending with one of ´p, ç, t, k´.

2. You want to add this word a suffix that starts with a vowel.

If the word has only one syllable, like saç, you are safe. The word usually does not change.

saç-ı --> saçı (his/her/its hair)

sap-a --> sapa (to the handle)

However, if the word has more than one syllable, than the consonant at the end usually changes.

• p becomes b
• ç becomes c
• t becomes d
• k becomes ğ

And here are some examples to this:

ağaç-a --> ağaca (to the tree)

şarap-ın --> şarabın (of the wine)

kağıt-a --> kağıda (to the paper)

geyik-e --> geyiğe (to the deer)

CASE B - Suffix mutation.

Two conditions must be satisfied for suffix mutation to occur:

1. You have a word ending with one of p, ç, t, k, f, h, s, ş.
2. You want to add this word a suffix that starts with c or d.
   
   

In this case, the first letter of the consonant changes.

• c becomes ç
• d becomes t

Examples:

Leh --> Polish (people)

Leh-ce --> Lehçe --> Polish (language)

Türk --> Turkish (people)

Türk-ce --> Türkçe --> Turkish (lanuage)

yap --> do

yap-di --> yaptı --> he did