Моя зачётка лежит в рюкзаке рядом со студбилетом.

Because зачётка is a feminine noun.

In Russian, possessives like мой / моя / моё / мои must agree with the noun they describe:

• мой = masculine
• моя = feminine
• моё = neuter
• мои = plural

So:

• моя зачётка
• мой студбилет
• моё место
• мои книги

Even though зачётка is a shortened, informal word, it still behaves like a normal feminine noun because it ends in -а.

Both are very common colloquial shortened words used by students.

• зачётка = short for зачётная книжка
  This is the student record book where passed courses/exams can be recorded.
• студбилет = short for студенческий билет
  This is a student ID card.

So the sentence sounds natural and everyday, especially in student speech. In more formal language, you might see the full forms instead.

Лежит comes from лежать, which literally means to lie or to be lying.

Russian often uses verbs like this for the physical position of an object:

• лежать = lie
• стоять = stand
• висеть = hang

So Моя зачётка лежит в рюкзаке means something like:

• My grade book is lying in my backpack
• or more naturally in English, My grade book is in my backpack

Russian prefers this kind of concrete verb more often than English does.

Yes, you could say:

• Моя зачётка находится в рюкзаке рядом со студбилетом.

But it sounds more formal, more neutral, and a bit less natural in everyday conversation.

Compare:

• лежит = natural, everyday, physical location
• находится = more formal, more like is located

So for an ordinary object in a backpack, лежит is the more idiomatic choice.

Because this sentence describes location, not motion.

After в, Russian uses:

• Accusative for motion into something
• Prepositional for being in something

So:

• кладу зачётку в рюкзак = I put the grade book into the backpack
• зачётка лежит в рюкзаке = the grade book is in the backpack

Here, the grade book is already there, so рюкзак changes to рюкзаке in the prepositional case.

Because it is in the prepositional singular after в when meaning in.

Basic pattern for many masculine nouns:

• nominative: рюкзак
• prepositional: в рюкзаке

Other examples:

• в столе
• в магазине
• в телефоне

So в рюкзаке simply means in the backpack.

After рядом you use с / со plus the instrumental case.

So the core pattern is:

• рядом с кем?
• рядом с чем?

The reason it is со, not just с, is mostly pronunciation. Russian often uses со instead of с before awkward consonant clusters.

Since студбилетом starts with сту-, рядом со студбилетом is easier to say than рядом с студбилетом.

This is similar to other examples like:

• со мной
• со столом

Because рядом с / со requires the instrumental case.

The noun студбилет is masculine, so in the singular instrumental it becomes:

• студбилет → студбилетом

Other examples:

• с другом
• с паспортом
• рядом с университетом

So:

• рядом со студбилетом = next to the student ID

The given word order is completely natural:

• Моя зачётка лежит в рюкзаке рядом со студбилетом.

Russian word order is fairly flexible, but changing it shifts emphasis.

For example:

• В рюкзаке рядом со студбилетом лежит моя зачётка.
  Emphasis on where it is
• Рядом со студбилетом в рюкзаке лежит моя зачётка.
  Stronger focus on next to the student ID

The original version is a neutral, straightforward statement.

In meaning, it describes the location of зачётка: the grade book is in the backpack, and inside the backpack it is next to the student ID.

So the structure is basically:

• лежит [в рюкзаке] [рядом со студбилетом]

Both phrases give location information. You can think of it as:

• It is in the backpack
• It is next to the student ID

Together: It is in the backpack, next to the student ID.

The correct spelling is зачётка, with ё.

However, in everyday writing and print, Russians often replace ё with е, so you may also see:

• зачетка

This does not usually change the meaning, but learners should know that the pronunciation is still with yo:

• зачётка = approximately za-CHYOT-ka

So when you read зачетка, you should still understand it as зачётка.

The main stresses are:

• моя́
• зачётка
• лежи́т
• в рюкзаке́
• ря́дом
• со студбиле́том

Stress is important in Russian because it is not always predictable. A learner might especially want to remember:

• зачётка
• рюкзаке́
• студбиле́том

The grammar is neutral, but the vocabulary is slightly informal/everyday because of the shortened student words:

• зачётка
• студбилет

A more formal version would be:

• Моя зачётная книжка лежит в рюкзаке рядом со студенческим билетом.

But in normal student speech, the short forms are very common and natural.