Защитная плёнка лежит на заднике.

Because it agrees with плёнка, which is a feminine singular noun in the nominative case.

The dictionary form of the adjective is защитный = protective.
Its forms change depending on the noun:

• защитный for masculine
• защитная for feminine
• защитное for neuter
• защитные for plural

So:

• защитная плёнка = protective film

Because плёнка is the subject of the sentence: it is the thing that is lying somewhere.

In Russian, the subject is usually in the nominative case.

So here:

• Защитная плёнка = the subject
• лежит = is lying
• на заднике = on the back panel / backing / rear side

Russian normally does not use a present-tense verb equivalent to English is in simple sentences like this.

Also, Russian often prefers a position verb when talking about where something is:

• лежать = to lie
• стоять = to stand
• сидеть = to sit
• висеть = to hang

So лежит does not just mean existence; it tells you the item is lying on a surface.

This is more natural in Russian than using something vague like is.

Because after на meaning on in a location sense, Russian usually uses the prepositional case.

Here the question is Where is it?
Answer: на заднике = on the back panel / on the backing

The noun is:

• dictionary form: задник
• prepositional singular: заднике

So:

• на заднике = on the back / on the backing / on the rear panel

Задник can mean different kinds of back part depending on context. It is not one of the most common everyday words for beginners, so it can feel a little unclear.

Possible meanings include:

• the back side of something
• the back panel
• the backing
• in some contexts, a backdrop

In this sentence, it most likely means the back part / rear side / backing of some object.

So the exact English translation depends on what object is being talked about.

Because the subject защитная плёнка is singular.

Russian verbs in the present tense agree with the subject in number and person:

• я лежу = I am lying
• ты лежишь = you are lying
• он / она / оно лежит = he / she / it is lying
• они лежат = they are lying

Since плёнка is singular, the verb is лежит.

Usually, yes. Лежать suggests that something is in a lying position, often resting on a surface.

For objects, Russian often chooses the verb based on how the object is positioned:

• книга лежит на столе = the book is lying on the table
• бутылка стоит на столе = the bottle is standing on the table
• картина висит на стене = the picture is hanging on the wall

So лежит suggests the protective film is resting there rather than standing or hanging.

Yes, but it changes the feel.

• Защитная плёнка лежит на заднике.
  = The protective film is lying on the backing/back panel.

• Защитная плёнка находится на заднике.
  = The protective film is located on the backing/back panel.

Находится sounds more neutral, formal, or impersonal.
Лежит sounds more concrete and natural if you are describing the physical position of the object.

Russian has no articles, so there is no direct equivalent of the or a/an.

Whether защитная плёнка means:

• a protective film
• the protective film

depends on the context.

Russian speakers understand this from the situation, previous mention, or word order if needed.

A natural stress pattern is:

• защи́тная
• плёнка
• лежи́т
• на за́днике

So the full sentence is pronounced approximately:

zash-CHEET-na-ya PLYON-ka lye-ZHEET na ZAD-ni-kye

A few useful notes:

• ё in плёнка is always pronounced yo
• ж sounds like the s in measure
• щит in защитная can be tricky for English speakers, so it may take practice

Yes. Russian word order is more flexible than English, although the basic version here is completely natural:

• Защитная плёнка лежит на заднике.

You could also change the order for emphasis, for example:

• На заднике лежит защитная плёнка.

This puts more focus on where it is.

The meaning stays basically the same, but the emphasis changes.