I have alluded to vowel harmony many times in my posts about Turkish, but I have not yet spelled out what it was. Partly, this was because when I mentioned it, I was still working out exactly what it is and how it works. But at this point I feel like I understand it, and there aren't any surprises waiting around the corner, so today I'm going to share a long overdue explanation.
In the simplest of terms, vowel harmony is just a nice way of letting you be a little bit lazy! It allows you to leave your mouth in a certain position for an entire word, rather than forcing you to reshape your mouth with every syllable.
Turkish vowels have three qualities that affect vowel harmony: front or back location, high or low location, and rounded or unrounded shape.
Front and back location of the vowel describes whether the vowel is formed at the front of the mouth or the back, and seems to be the most significant detail in vowel harmony. Front-vowels are i, e, ö, and ü. Back-vowels are ı, a, o, and u.
High and low location of the vowel describes whether it is formed high in the mouth or low. High vowels are: i, ü, ı, and u. Low vowels are: e, ö, a, and o.
Rounded and unrounded describes the shape of the mouth as the vowels are formed. Rounded vowels are: o, u, ö, and ü. Unrounded vowels are: a, ı, e, and i.
There are two groups of vowels that get vowel harmony. The first is the pair of unrounded, low vowels: a and e. These were easiest to figure out by observing the infinitive endings -mak and -mek, the plural endings -lar and -ler, the prepositional endings -da and -de.
The second group consists of the four high vowels: i, ı, u, and ü. They can be seen in all of the conjugated endings, such as -ım, -im, -um, and -üm or -siniz, -sınız, -sunuz, and -sünüz.
In both cases, the key ingredient is whether or not the preceding vowel was a front vowel or a back vowel, though in the case of the second group we also consider roundedness.
So... That all reads like a complicated bunch of rules and that's not fun. But vowel harmony really isn't hard, and after just a little bit of attention, it starts to make a lot of sense.
I actually prefer to think of Turkish vowels as groups, rather than single letters, so in my mind, for all types of turkish endings, the vowels are: [a,e] and [i,ı,u,ü]. Thus, the plural ending is -l[a,e]r, the prepositional ending is -d[a,e], the first-person singular ending is -[u,ü,i,ı]m, and so on.
And actually, you don't even have to remember what the choices are. All you have to do is make the sound that your mouth is already positioned to make!
When you see all laid out, it's much clearer. In the following examples, the preceding vowel is underlined, and the harmonized vowel is bold.
Here are a few examples of plurals:
Can you say them incorrectly? Sure. Is it difficult? No. But still, if you pay attention to your mouth as you say gecelar (wrong) and geceler (right) you can feel a difference. The second feels more comfortable, and also sounds more pleasant.
Here are a few more examples, using verb conjugations:
That last word is perhaps the best example. I notice that if I try to say konuşiyor (wrong), or konuşıyor (wrong), it still manages to come out sounding like konuşuyor (right), because my mouth is already positioned to make that sound.
So that's the key: Rather than thinking of vowel harmony as some difficult rule that has to be followed, it's best to think of it as a benefit — it's an adjustment to make the language easier for the speaker. I think that's pretty cool.