"किताब नीली है।"

Translation: The book is blue.

[keerthanakithu._]

Why the answer 'Book is blue' not accepted???

Why 'the' is compulsory?

[Shaunak296]

Because you are talking about a specific book. If the sentence was about books in general, it would be "Books ARE blue."

[ExclusiveB1]

How come the book is blue in colour be wrong

[kati.paul]

Because the "in colour" bit isn't in the Hindi sentence. I think that it is sort of required in some varieties of English, tho, so I see why you added it!

[Nivetha946498]

Why is book femine

[shrikrishna1]

Because it is!

[Rodrigo188394]

Besides memory learning adjective gender , is there some sort of help somewhere ?

[WDgDns5g]

Mostly masculine ends in "aa" - beta (remember there are always exceptions as in any languages in general) Examples: Yamuna (f), Ganga (f) rivers
and feminine ends in "ee" = beti

https://blogs.transparent.com/hindi/masculine-and-feminine-nouns-in-hindi/