The letter "ی" is the 30th letter of the Persian alphabet. It is pronounced like the English letter "Y" and can represent both a consonant and a vowel. In Persian, "ی" is often used at the end of words to indicate a long vowel sound, similar to the "ee" in "see." In addition to its phonetic role, "ی" is also used in various grammatical forms in the Persian language. For example, it can serve as a possessive marker, indicating ownership, as in the word "کتاب" (book) when combined with "ی" to form "کتابی" (my book).