Стоп! Дверь закрыта.

In Russian, the verb to be is normally left out in the present tense.

So:

• Дверь закрыта = The door is closed
• literally, something like Door closed

This is completely normal Russian.
If you want past or future, then forms of быть appear:

• Дверь была закрыта = The door was closed
• Дверь будет закрыта = The door will be closed

Закрыта is the short form, while закрытая is the full form.

In sentences like Дверь закрыта, Russian usually uses the short form to express a state:

• Дверь закрыта = The door is closed

The full form закрытая is more like an attributive adjective, used before a noun or in a more descriptive way:

• закрытая дверь = a closed door

So:

• закрытая дверь = a closed door
• дверь закрыта = the door is closed

Because it agrees with дверь, and дверь is a feminine singular noun.

Short-form adjectives/participles agree in gender and number:

• masculine: закрыт
• feminine: закрыта
• neuter: закрыто
• plural: закрыты

Examples:

• Магазин закрыт = The shop is closed
• Дверь закрыта = The door is closed
• Окно закрыто = The window is closed
• Двери закрыты = The doors are closed

You mainly have to learn it as part of the noun.

Even though дверь ends in a soft sign ь, it is feminine. Many nouns ending in ь are feminine, though not all of them.

A good habit is to memorize nouns with their gender:

• дверь — feminine
• словарь — masculine
• ночь — feminine
• день — masculine

You can see that дверь is feminine here because the predicate is закрыта, the feminine form.

For a learner, the most useful answer is: in this sentence, it behaves like a short-form predicate word meaning a state.

Historically, forms like закрыт / закрыта / закрыто / закрыты come from a short passive participle. In modern Russian, learners often just treat them as the normal form used in sentences like:

• Дверь закрыта
• Окно открыто
• Магазин закрыт

So you do not need to worry too much about the label at first. The key point is that this form is commonly used after the noun to mean is closed/open/etc.

It is in the nominative case.

In Дверь закрыта, дверь is the subject of the sentence:

• дверь = the door
• закрыта = is closed

So this is a basic subject + predicate structure.

Yes, Стоп! exists in Russian and is understandable and common in many situations. It is a borrowing from English, but it is fully familiar in Russian.

It can sound like:

• Stop!
• Wait!
• Hold it!

Depending on context, Russian might also use other words:

• Стой! = Stop! (to one person, informal)
• Стоять! = Stop! / Freeze!
• Подожди! = Wait!
• Остановитесь! = Stop! (formal/plural)

So Стоп! Дверь закрыта. is natural, especially in a short warning-style context.

No. Those forms do not agree correctly with дверь.

Because дверь is feminine singular, you need:

• Дверь закрыта

Not:

• Дверь закрытый ❌
• Дверь закрыто ❌

Compare:

• магазин закрыт = shop, masculine
• дверь закрыта = door, feminine
• окно закрыто = window, neuter

Agreement is very important in Russian.

Yes, Russian word order is more flexible than English, but the most neutral version here is:

• Дверь закрыта.

Other orders are possible in context, for example:

• Закрыта дверь.

That version is less neutral and may sound more emphatic, poetic, or context-driven. For a learner, Дверь закрыта is the best basic form to use.

They are related, but they do not mean the same thing.

• Дверь закрыта = The door is closed
  This describes the state/result.

• Дверь закрылась = The door closed / The door shut
  This describes the event/action happening.

So:

• if you want to say what condition the door is in now, use Дверь закрыта
• if you want to say that the door changed from open to closed, use Дверь закрылась

Sometimes, depending on context, yes. But the most direct meaning is simply The door is closed.

Russian short forms like закрыта often focus on the resulting state, not on who did the action. So English may translate it as:

• The door is closed
• sometimes, in context, The door has been closed

If you specifically want to mention the action by someone, Russian often uses a fuller passive structure or a different wording.

A rough English-friendly guide is dvyer’.

A few important points:

• дв is pronounced together
• е after a soft consonant sounds like ye
• the final ь (soft sign) is not pronounced as a separate sound, but it makes the final consonant soft

So the word is not like plain dver with a hard ending. The end is softened.

If you want a rougher approximation:

• дверь ≈ dvyer'

Because the second sentence is simply a statement: The door is closed.

The urgency is already carried by Стоп!.
Of course, in real writing you might also see:

• Стоп! Дверь закрыта!

if the speaker is being especially forceful or emotional. But the version with a period is perfectly normal and neutral.