Questions & Answers about किताबों की अलमारी बड़ी है।
Because किताब is in the plural oblique form here.
- किताबें = plural direct form
- किताबों = plural oblique form
In Hindi, a noun usually changes to the oblique form when it comes before a postposition such as का / की / के, में, से, etc.
Here, किताबों comes before की, so it must be oblique:
- किताबें = books
- किताबों की अलमारी = book cupboard / bookcase / cupboard for books
So this is a very common pattern:
- लड़के = boys
- लड़कों का स्कूल = the boys’ school