"ياء" is the Arabic letter that represents the sound /j/ as in "yes." It is the 10th letter of the Arabic alphabet and is written in different forms depending on its position in a word: isolated, initial, medial, or final. In its isolated form, it appears as ي. In addition to its phonetic role, "ياء" can also function as a grammatical marker in Arabic. For example, it is used as a suffix to indicate possession, as in the word "كتابك" (your book), where "ك" is a possessive pronoun. This versatility makes "ياء" an essential component of the Arabic language.