Мешок лежит у двери.

Because the preposition у normally requires the genitive case.

• Dictionary form: дверь = door
• Genitive singular: двери

So:

• у двери = by the door / near the door

This is very common in Russian:

• у дома = by the house
• у окна = by the window
• у стола = by the table

In this sentence, двери is not plural. It is singular genitive.

Here у means by, near, or next to.

So у двери means:

• by the door
• near the door
• next to the door

It usually shows location close to something.

Be careful: у can also mean at someone's place or in someone's possession in other contexts, for example:

• Я у друга. = I am at my friend's place.
• У меня есть книга. = I have a book.

But in Мешок лежит у двери, it simply means near the door.

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

Russian often uses different verbs depending on the object's position:

• лежать = to lie, to be in a horizontal position
• стоять = to stand, to be upright
• висеть = to hang

A мешок (bag / sack) is naturally thought of as something lying on the floor, so лежит is very natural.

So this sentence does not just say that the sack is there. It also suggests its physical position: it is lying there.

Usually no, or at least it would sound different.

• Мешок лежит у двери = the sack is lying by the door
• Мешок стоит у двери = the sack is standing by the door

A sack is normally imagined as lying down, so лежит is the normal choice.

You might hear стоит only if the bag is upright and the speaker wants to describe it that way. Russian pays attention to posture/position more than English often does.

In the present tense, Russian usually does not use a verb meaning is/am/are in simple statements like this.

English:

• The sack is by the door.

Russian:

• Мешок у двери.
• or more specifically: Мешок лежит у двери.

So Russian often leaves out the present-tense to be.

Here, instead of using is, Russian uses лежит, which already gives the idea of both is located and is lying.

Because мешок is the subject of the sentence, so it is in the nominative case.

• мешок = nominative singular
• It answers the question What is lying by the door?
• Answer: мешок

So the sentence structure is:

• Мешок = subject
• лежит = verb
• у двери = location

No. In this sentence, двери is singular, not plural.

That can be confusing, because двери can also be a plural form in other contexts.

Here it is:

• дверь = nominative singular
• двери = genitive singular

Since у requires the genitive, у двери must mean by the door, not by the doors.

Yes. Russian word order is more flexible than English word order.

The neutral order here is:

• Мешок лежит у двери.

But other orders are possible, for example:

• У двери лежит мешок.
• Лежит мешок у двери.

These versions still mean basically the same thing, but the emphasis changes:

• Мешок лежит у двери. = neutral, starting with the sack
• У двери лежит мешок. = emphasizes the location first: by the door, there is a sack
• Лежит мешок у двери. = more marked, often stylistic or context-driven

So word order in Russian often affects focus, not just grammar.

The stress is:

• мешо́к
• лежи́т
• у
• две́ри

So the full sentence is pronounced:

мешо́к лежи́т у две́ри

Stress is important in Russian because it can change pronunciation and sometimes meaning.

Yes, in many situations that is a natural English translation.

But Russian is slightly more specific:

• лежит tells you the sack is lying
• у двери tells you it is near/by the door

So depending on context, possible English translations include:

• The sack is by the door.
• The sack is lying by the door.
• The sack is near the door.
• The sack is at the door.

If the meaning has already been given to the learner, the main extra thing to notice is that Russian often includes the object's position more explicitly than English does.

Yes, около двери could also work and means near the door.

Compare:

• у двери = by the door / right near the door
• около двери = near the door

Very often they are close in meaning, but у двери can feel a bit more like right by the door.

So:

• Мешок лежит у двери.
• Мешок лежит около двери.

Both are correct, but у двери is very natural and compact.