Learn Hardcore Russian: В её рюкзаке всегда лежит книга, вода и привычная тетрадь. - In her backpack there is always a book, water, and a familiar notebook.

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)

Why does её look the same here? Shouldn’t it change its ending for the case?

Её is a special kind of possessive pronoun: it is indeclinable.

That means its form does not change for:

case (nominative, genitive, dative, etc.),
gender (masculine/feminine/neuter),
number (singular/plural).

So you always see её:

её рюкзак (her backpack)
в её рюкзаке (in her backpack)
с её подругой (with her friend, feminine)
с её братом (with her brother, masculine)

In contrast, adjectives and some other possessives do change endings, but её does not.

Why is the verb singular (лежит) if there are three things: книга, вода и привычная тетрадь?

Grammatically, we have a compound subject (three nouns joined by и) and a verb:

Subject: книга, вода и привычная тетрадь
Verb: лежит

“Classical” agreement would be plural:

В её рюкзаке всегда лежат книга, вода и привычная тетрадь.

However, Russian often allows a singular verb when:

the verb comes before the list of subjects,
and the speaker treats the list as one set / one group of items.

So both are possible:

лежат – more straightforward grammatically; emphasizes three separate items.
лежит – somewhat more colloquial/neutral; feels like “there is (lying) a set of things: a book, water, and a familiar notebook.”

In formal written Russian, many teachers and editors would recommend лежат here, but лежит is very commonly heard and read.

What case are книга, вода, тетрадь in, and why?

They are in the nominative case because they are the grammatical subject of the sentence.

Structure:

В её рюкзаке – a prepositional phrase indicating location (“in her backpack”)
всегда лежит – the predicate (“always lies/there is lying”)
книга, вода и привычная тетрадь – the subject (what is lying there)

In Russian, the subject is typically in the nominative:

Книга лежит на столе. – The book lies on the table.
В комнате стоит стол. – There is a table standing in the room.

So:

книга, вода, (привычная) тетрадь are nominative subjects,
рюкзаке is prepositional, part of the location phrase.

Why do we use лежит instead of something like есть or находится?

Russian has a family of “verbs of position” used instead of a generic “to be” in many contexts:

лежать – to lie (be in a lying position)
стоять – to stand
сидеть – to sit
висеть – to hang

Here:

лежит literally means “is lying (there)”, but often just means “is / there is” for objects placed horizontally (like a book, a notebook, a bottle of water).

Compare:

В её рюкзаке всегда лежит книга…
Focuses slightly on placement/position (“they are lying there inside”).
В её рюкзаке всегда есть книга…
More abstract “there is always a book in her backpack”, without the physical image of things lying.
В её рюкзаке всегда находится книга…
More formal / technical, “is located / is found”.

In everyday speech, лежать is very natural for small items inside bags, on tables, on shelves, etc.

Why is привычная тетрадь in that exact form? What does привычная mean and how does it agree with тетрадь?

Привычная comes from привычка (“habit”) and means:

привычный (masc.) / привычная (fem.) = usual, familiar, habitual

So привычная тетрадь ≈ “her usual/familiar notebook” – the one she always uses, the one she is used to.

Grammatically:

тетрадь is feminine (even though it ends in a soft sign ь).
The adjective must agree with it in:
- gender: feminine,
- number: singular,
- case: nominative.

Nominative feminine singular of this adjective is привычная:

привычная тетрадь (nom. fem. sg.)
привычной тетради (gen. or dat. fem. sg.), etc.

So the form привычная is there to match тетрадь as the third nominative subject.

Can the word order be changed, for example to Книга, вода и привычная тетрадь всегда лежат в её рюкзаке? Does the meaning change?

Yes, you can change the word order, and the basic meaning stays the same.

Your original:

В её рюкзаке всегда лежит книга, вода и привычная тетрадь.
Structure: location → frequency/adverb → verb → subject.

Another version:

Книга, вода и привычная тетрадь всегда лежат в её рюкзаке.
Structure: subject → frequency/adverb → verb → location.

Nuances:

Starting with В её рюкзаке… puts emphasis on the place – what is in her backpack.
Starting with Книга, вода и привычная тетрадь… emphasizes which items always lie there.

Both orders are natural. Russian word order is relatively flexible; it is often used to shift focus rather than to mark grammar.

Why is there a comma after книга but not after вода?

This is about how lists are punctuated in Russian.

In a simple list of three items with и (“and”) before the last one:

you put commas between all items except the last pair connected by и.

So:

книга, вода и привычная тетрадь
“a book, (some) water and a familiar notebook”

Rules:

Between книга and вода → comma.
Between вода and привычная тетрадь → the conjunction и, no comma.

Russian does not use an English-like “Oxford comma” before и in this structure. You would not write:

✗ книга, вода, и привычная тетрадь (this looks wrong in Russian).

Why is её only used once (before рюкзаке) and not repeated before each item like её книга, её вода и её привычная тетрадь?

In Russian, when several nouns belong to the same person, you usually use the possessive once and let it apply to the whole group, unless you need special emphasis or contrast.

So:

В её рюкзаке всегда лежит книга, вода и привычная тетрадь.
Is naturally understood as: in her backpack there is (her) book, (her) water, and (her) usual notebook.

You could say:

В её рюкзаке всегда лежат её книга, её вода и её привычная тетрадь.

But that sounds:

stylistically heavier,
like you are stressing her book, her water, etc., possibly in contrast to someone else’s things.

In neutral sentences, one её is enough.

What does всегда do here, and can it be moved or removed?

Всегда means “always”, and it modifies the verb лежит:

В её рюкзаке всегда лежит…
= “There is always lying in her backpack…”

You can move всегда around a bit without changing the core meaning:

В её рюкзаке лежит всегда книга, вода и привычная тетрадь.
Possible, but a bit marked; may put slight rhythmic/contrastive emphasis on лежит всегда.
Всегда в её рюкзаке лежит книга, вода и привычная тетрадь.
Emphasizes “Always, in her backpack there is…” (stronger focus on the constancy).

If you remove всегда:

В её рюкзаке лежит книга, вода и привычная тетрадь.
Now it just states what is in her backpack, not that it is always there (no habitual/constant nuance).

Is there any difference in meaning between В её рюкзаке всегда лежит… and У неё в рюкзаке всегда лежит…?

Both are grammatical and common, but they focus slightly different things.

В её рюкзаке всегда лежит…
- Literally: “In her backpack there always lies…”
- Focus on the object (the backpack) and what is inside it.
- The possessive её is attached directly to рюкзак.
У неё в рюкзаке всегда лежит…
- Literally: “By her, in [her] backpack there always lies…”
- У неё is a dative/possessive-like construction meaning “she has” / “with her”.
- Feels more like “she always has in her backpack…” (emphasis on her possession or habit).

So:

В её рюкзаке… – more about the location (in that backpack).
У неё в рюкзаке… – more about her having these things with her.