Why not Yah kitab acchi he?
That would be 'This book is good'.
The placement of adjectives in Hindi is similar to English. Attributive adjectives go before the noun ('अच्छी किताब'/'good book') while predicative adjectives go after the noun ('किताब अच्छी है' / 'book is good')
Why not अच्छा kithab...?
किताब is a feminine noun.
Book is feminine in hindi
Do i have to use एक
Duo says it's wrong
Dropping the एक is fine. You can report if यह अच्छी किताब है। is not accepted.