Hi Ahmed. This is the book that is blue. این کتابی است که آبی است This is a book that is blue. این یه کتاب است که آبی است (این کتابی است که آبی است) ی is not needed for the second sentence and یه is fine. But as you see you can write the second sentence in another way by using ی when your noun is indefinite however it is less colloquial compared to یه. then the difference between the two Persian sentences in the case of using ی would be the stress. there are two stresses, one in the first clause of two sentences : in "this is the book ..." , the stress is on "این " and in "this is a book ..." the stress is on "کتاب". Also, there is another stress between the whole two sentences. In the sentence "This is the book that is blue" the general stress is on the first clause rather than emphasizing the color but in the sentence "This a book that is blue" the general stress is on the second clause which is color. Last point: when ی is used as the indefinite marker it is unstressed.