As you will see more in depth in the following sections, the following rules apply with regards to vowels and consonants in Turkish.
Turkish words are made up as: stem + suffix + suffix + ...
For vowels: What a vowel in a stem or a suffix is allowed to be, is determined by what the previous vowel is in the word. The vowels in the stem are fixed (never change), but the vowels in a suffix may become one of 2 or 4 possible choices depending on what the previous vowel is. Example: “e” (to) becomes a “a” after a syllable with an “a”: ev+e (to home), av+a (to hunting).
For consonants: When a suffix is appended to a stem or to the previous suffix, Turkish tries to prevent a soft consonant and a hard consonant from following each other. To provide for this rule, the last consonant of the front part or the first consonant of the last part may change. Example: suffix “de” (in/on/at) becomes “te” after a word ending in a hard consonant: ev+de (at home), iş+te (at work).
İstiklal caddesi, İstanbul