Originally posted by: MeteUlku with a little help of Ektoraskan https://forum.duolingo.com/comment/13722700
In Turkish, it is called as "Ünlü Düşmesi". While using some words in Turkish, we sometimes need to drop their last vowel.
Someone asked this in a sentence discussion topic. I thought to create this little guide for people who have a trouble with this 
Good luck with that.
Let's get to know some words that need to be cut:
- Ağız (mouth)
- Oğul (son)
- Alın (forehead)
As in the example words, their last letters are consonants and vowels come before them. If we want to create a possessive sentence, we will drop the LAST vowel which is coming BEFORE the consonant.
Oğlum (my son)
Ağzım (my mouth)
Alnım (my forehead)
(Onun) oğlu (his/her son)
Ağızım / oğulum / alınım (INCORRECT)
As we see, when we use possesive nouns, the last vowel drops out of the sentence. And because there is a possessive, the noun takes -ım. Why -ım? Because it is a possesive suffix for 1st person singular.
If we don't comply with this rule, the sentences we created will be unnaturally incorrect. So, we had better to comply with this
If you have any cruiosity about this topic, let me/us know and try to answer that.
Good luck with Turkish
An informative writing from Ektoraskan about this:
A couple of notes;
It doesn't only happen in possessives. It also occurs in the accusative, dative and genitive: ağzı, ağza, ağzın.
In order for this phenomenon to happen, the word must have the following characteristics:
- The word must have exactly two syllables; not more, not fewer.
- The word must be in the (C)VCVC format, where C is a consonant and V is a vowel.
- The last vowel must be ı, i, u, ü. It cannot be a/e/o/ö.
Observe the following words and check how they comply with the above three rules: "ağız, resim, omuz, karın, şehir, vakit, nakit, ufuk\".\n\n> Important note: All the words that lose their vowel must satisfy these three rules, but not every word that satisfies these rules loses their vowel. So that's something to keep in mind. For example: "umut, durum, gelin, delik" all comply with the rules but don't lose their vowel.