Turkish Consonant Softening and Assimilation

Where a suffix joins a word, the last letter of the word and the first letter of the suffix meet. Two changes happen at that seam, and both are about what is easy to say.

Softening (yumuşama): when a word ends in p, ç, t or k and the suffix starts with a vowel, that letter softens to b, c, d or ğ: kitap + ı → kitabı, ekmek + i → ekmeği. The spelling changes because the sound does.

Hardening (sertleşme): when a word ends in one of f, s, t, k, ç, ş, h, p (the letters of fıstıkçı şahap) and the suffix starts with d or c, the suffix hardens to t or ç: kitap + da → kitapta, süt + cü → sütçü. Look at the first letter of the suffix: a vowel means softening may apply, d or c means hardening may apply. They never apply together.