Questions & Answers about ده نفس الكتاب.
ده is the Egyptian Arabic masculine singular demonstrative. It usually means this or that.
At the start of this sentence, ده works like this is or that is. So ده نفس الكتاب means This/That is the same book.
Because كتاب is masculine singular, ده is the right form here.
In Arabic, the present-tense verb to be is usually omitted.
So where English says This is the same book, Egyptian Arabic just says:
ده نفس الكتاب
Literally, it is closer to this same the-book.
Because ده is being used as a standalone demonstrative pronoun here.
There is an important difference:
الكتاب ده = this book
This is a noun phrase.
ده نفس الكتاب = this/that is the same book
This is a full sentence.
So when ده comes first, it is not modifying الكتاب directly. It is the subject of the sentence.