One question that plagues many new students of the Arabic language is the difference between hadhaa kitaabun and hadhaal kitaabu …
The difference is best demonstrated by example: hadhaa kitaabun kabiyrun means “this is a big book,” and “hadhaal kitaabul kabiyru” means “this big book …”
Get the difference? The first (hadhaa kitaabun) is talking about any book. “This is a book.” Nothing special. The second–hadhaal kitaabu–is talking about a specific book. It’s saying “this book … something.”
And how do you know the difference? in the second case, the book is definite–it’s a specific book. Notice, al-kitaabu–the book. Not kitaabun–a book.
And, notice the second one is a fragment–this big book what? This big book is open? This big book is on the table? And so on.