Learn Hardcore Ukrainian: Якщо ліфт зупиниться, натисни кнопку праворуч. - If the elevator stops, press the button on the right.

Ukrainian often uses the future perfective to talk about a single, potential event in an if-clause. Зупиниться is the perfective future of зупинитися “to stop (come to a stop)” and fits a one-time malfunction: “If the elevator stops (at some point) …”.
Use the imperfective present for a habitual scenario: Якщо ліфт зупиняється, натискай кнопку праворуч. “If the elevator tends to stop, keep pressing the button on the right.”

What form is натисни, and how would I say it politely?

Натисни is the informal singular imperative (“press!” addressed to one person you know).
Polite/plural imperative: Натисніть.
You can add politeness with будь ласка: Будь ласка, натисніть кнопку праворуч.

Why is it кнопку and not кнопка?

Кнопку is the accusative singular of the feminine noun кнопка. The verb “press” takes a direct object in the accusative: “press what? the button.”

Where is “the” in “the button”? Ukrainian has no articles, right?

Correct—Ukrainian has no articles. Definiteness is understood from context. Here, кнопку праворуч (“the button on the right”) is clearly a specific button.

Why use праворуч? Could I say something else for “on the right”?

Yes. Common options:

праворуч = on the right/to the right (neutral, often used in instructions).
справа = on the right (very common in speech).
праву кнопку = the right(-hand) button (using the adjective): натисни праву кнопку. Left-side counterparts: ліворуч, зліва, ліву кнопку.

Is it better to say натисни кнопку or натисни на кнопку?

Both are acceptable. Without the preposition (натисни кнопку) is slightly more formal/concise; with на (натисни на кнопку) is very common in everyday speech and emphasizes pressing onto the surface.

Can I change the word order? For example, put the location first?

Yes, but keep it natural:

Standard: Натисни кнопку праворуч.
Also fine: Натисни праворуч кнопку. (less common, still acceptable)
Very natural alternative with an adjective: Натисни праву кнопку. Avoid splitting the noun and its clarifier too awkwardly.

What’s the difference between якщо and коли here?

Якщо = if (a condition that may or may not happen): Якщо ліфт зупиниться…
Коли = when (a time that is expected/certain): Коли ліфт зупиниться, натисни… implies the stop is expected.

Do I need a word like “then” in the main clause—то or тоді?

It’s optional. You may say: Якщо ліфт зупиниться, тоді натисни кнопку праворуч. The meaning is the same; тоді/то just adds an explicit “then.”

Why is there a comma after the first clause?

Ukrainian uses a comma to separate a subordinate clause from the main clause. If you reverse the order, you still use a comma: Натисни кнопку праворуч, якщо ліфт зупиниться.

Why is the verb “to stop” reflexive (зупинитися)? What does -ся mean?

The clitic -ся/-сь marks reflexive/mediopassive meanings. Many intransitive or self-contained actions use it by convention. Зупинитися literally “to stop oneself; to come to a stop.” In 3rd person future it becomes зупиниться (ending -ться).

Could I say “gets stuck” instead of “stops”?

Yes: Якщо ліфт застрягне, натисни кнопку праворуч.

зупиниться = stops (movement ceases)
застрягне = gets stuck (jammed)

Is нажми okay for “press”?

In standard Ukrainian, prefer натисни (perfective) or тисни (imperfective). Нажми is widely understood but considered colloquial and influenced by Russian; avoid it in careful speech or writing.

What’s the difference between натисни and натискай?

Натисни (perfective imperative) = do it once.
Натискай (imperfective imperative) = press repeatedly/keep pressing, or a general instruction habitually.

Is ліфт the usual word? What about підйомник?

Ліфт is the standard word for an elevator in a building. Підйомник is a lifting device/hoist (e.g., ski lift, workshop lift) and is not used for a passenger elevator in everyday speech.

How do I pronounce or transliterate the sentence?

A common transliteration: Yakshcho lift zupynytsia, natysny knopku pravoruch.
Notes:

щ = “shch” (as in “fresh cheese” said quickly).
и is a central vowel (not like English “ee”); і is like English “ee.”
ць in …ниться sounds like “ts” with slight softening before “a” (roughly “tsia”).