Dropped vowels: ağız → ağzı

Some two-syllable nouns lose the vowel of their last syllable when a suffix that starts with a vowel is added. The lost vowel is always ı, i, u or ü:

• ağız → ağzı (his/her mouth), burun → burnu, alın → alnı, boyun → boynu, göğüs → göğsü, karın → karnım, beyin → beyni
• oğul → oğlu (his/her son), isim → ismin (your name), resim → resmi
• şehir → şehri, nehir → nehri, fikir → fikrim, akıl → aklı, sabır → sabrım, keyif → keyfi, gönül → gönlü, ömür → ömrü

Many of them are body parts or abstract words. Before a suffix that starts with a consonant the vowel stays: ağızda, şehirler, isimsiz. Only a limited group of words behaves this way, so they have to be learned one by one: kitap, kalem or kadın never lose a vowel.