Breakdown of В её рюкзаке всегда лежит книга, вода и привычная тетрадь.
Questions & Answers about В её рюкзаке всегда лежит книга, вода и привычная тетрадь.
The choice of ending is about case and type of movement.
When в means “in / inside” with no movement, it normally takes the prepositional case.
- рюкзак (dictionary form, nominative) → в рюкзаке (prepositional: “in the backpack”)
When в means “into” (movement to a place), it takes the accusative:
- в рюкзак = “into the backpack” (putting something in)
In your sentence, the meaning is that the book, water, and notebook are located inside the backpack, not moving into it. So we use the prepositional form рюкзаке:
- В её рюкзаке = “In her backpack” (location)