Відкрийте рот.

Because Відкрийте is an imperative form: it means open!

The ending -йте here marks:

• a command to more than one person, or
• a polite/formal command to one person.

So Відкрийте рот. is the polite/plural way to say Open your mouth.

The base verb is відкрити = to open.

The informal singular version is:

Відкрий рот.

Use this when speaking to:

• one friend,
• a child,
• someone you address with ти.

Use Відкрийте рот. for:

• one person formally (ви),
• or several people.

In Ukrainian, words like my, your, his, etc. are often left out when the meaning is obvious.

So Відкрийте рот. literally looks like Open mouth, but it naturally means Open your mouth.

This is very common with:

• body parts,
• clothing,
• personal items, when context makes ownership obvious.

For example, Ukrainian often says the equivalent of:

• Raise hand
• Close eyes rather than always saying your hand, your eyes.

It is the direct object, so it is in the accusative case.

But for many inanimate masculine nouns, the accusative looks the same as the nominative.

So:

• nominative: рот = mouth
• accusative: рот = mouth (same form)

That is why you do not see a change here.

Yes. Рот is the normal everyday word for mouth.

It is used in sentences like:

• Відкрийте рот. = Open your mouth.
• Закрий рот. = Shut your mouth / Close your mouth.

There is also уста, but that is much more literary, poetic, or elevated in style, not the normal word in this kind of instruction.

Yes. Відкрити is a perfective verb.

That means it focuses on a single completed action: open it.

This is very natural in commands like this, because the speaker wants one clear result:

• Open your mouth.

The imperfective partner is відкривати = to be opening / to open repeatedly / in general.

Its imperative is відкривайте. That form can be used too, but Відкрийте рот. is the most natural simple instruction for one completed action, such as at the doctor or dentist.

A helpful approximation is:

vid-KRYY-te rot

A few points:

• Stress falls on -крий-: відкри́йте
• р is rolled or tapped
• Ukrainian и does not sound exactly like English ee or ih; it is a short central vowel
• о in рот is a clear o sound

So:

• Відкри́йте = vid-KRYY-te
• рот = rot

Yes, Ukrainian word order is fairly flexible.

The neutral, standard order is:

Відкрийте рот.

But other orders are possible for emphasis, for example:

• Рот відкрийте.

That can sound more marked or emphatic, depending on context.

For a learner, the best default is:

• Відкрийте рот.

Yes. Відкрийте is polite if you are speaking to one person formally, and it is also the form for addressing more than one person.

So this is exactly what you might hear from:

• a doctor,
• a dentist,
• a teacher speaking politely,
• any formal instruction.

It is direct because it is an imperative, but it is not rude by itself.

It depends on the situation.

In many real-life contexts, especially medical ones, a simple imperative is perfectly normal:

• Відкрийте рот.

If you want to sound softer or more explicitly polite, you can add будь ласка:

• Відкрийте рот, будь ласка. = Open your mouth, please.

Both are natural.

Because Ukrainian has no articles.

So Ukrainian does not have direct equivalents of English a and the.

That means:

• рот can mean a mouth, the mouth, or just mouth depending on context.

In this sentence, the natural English translation is Open your mouth, even though Ukrainian does not literally include your or any article.

Normally, no — not in standard usage.

Відчинити / відчиніть is usually used for things like:

• doors,
• windows,
• gates.

For a mouth, book, eyes, etc., відкрити / відкрийте is the normal verb.

So:

• Відкрийте рот. = natural
• Відчиніть рот. = not the usual choice

Yes, absolutely. This is a very common real-life instruction in medical settings.

A doctor or dentist might say:

• Відкрийте рот.
• Скажіть “а”. = Say “ah.”
• Закрийте рот. = Close your mouth.

So this is a very practical sentence to know.