Originally posted by: AlexinNotTurkey https://forum.duolingo.com/comment/9041808
In Turkish, vowels within a (native) word and any suffixes that are attached to said word must obey vowel harmony rules. This means that vowels tend to either be the same or similar, making words easier to pronounce than they may look.
There are two types of vowel harmony in Turkish, 4-way and 2-way. Vowel harmony is one of the biggest aspects of Turkish phonology and it is fundamental part of learning the language. With practice, you will start to be able to do much of this work without thinking.
The accusative case is one of many suffixes that uses 4-way vowel harmony. We will use this case as our example, but remember that it pops up in many other places and will be represented with a -I (capital i) in our course.
In order to figure out what may go on the end, you will have to look at the final vowel in the word.
 that do not follow vowel harmony. Some of these words are quite common, and any good dictionary will tell you that they are exceptions (keep in mind, many online dictionaries are not good). Some examples of this are: Saat (hour/clock) - Saati (acc.) // Terminal (station) - Terminali (acc.) // Alkol (alcohol) - Alkolü (acc.)