Если счёт оплачен, оставь чаевые на столе.

In Russian, a subordinate clause introduced by если (if) is normally separated from the main clause by a comma.
So Если счёт оплачен (the condition) + comma + оставь чаевые на столе (the instruction).

счёт means the bill/check (in a restaurant) or an account (bank account), depending on context. Here it’s the restaurant bill.
Pronunciation: счёт is stressed on the only syllable, and ё is pronounced yo: roughly shchyot.

оплачен is a short passive participle meaning (it is) paid / has been paid—it focuses on the resulting state: the bill is already settled.
Compare:

• Счёт оплачен. = The bill is paid (status/result).
• Счёт оплатили. = They paid the bill (focus on the action/actor).

Short passive participles agree in gender and number with the noun:

• счёт is masculine singular → оплачен (masc. sg.) Other patterns:
• сумма оплачена (fem. sg.)
• письмо оплачено (neut. sg.)
• счета оплачены (plural)

It’s optional. то can be added for emphasis or clarity, especially in longer sentences:

• Если счёт оплачен, (то) оставь чаевые на столе. In short everyday sentences, то is often omitted.

This is about aspect in the imperative:

• оставь (perfective) = leave (it) / do it (once), achieve the result
• оставляй (imperfective) = be leaving / leave habitually / keep doing it (often sounds like repeated action or general advice) Here, leaving a tip is a one-time completed action, so оставь is natural.

Russian commonly drops subject pronouns because the verb ending shows the person:

• оставь = you (informal singular) leave If you want to be explicit (often for contrast), you can add ты, but it’s not necessary.

Yes. оставь is addressed to one person informally (to a friend, family member, etc.).
Polite/plural:

• Если счёт оплачен, оставьте чаевые на столе. = (Please) leave a tip on the table.

чаевые is a plural-only noun in Russian (like “earnings” in form), meaning tip/tips.
You normally don’t use a singular form to mean a tip. You can quantify it:

• оставь чаевые = leave a tip
• оставь немного чаевых = leave some tip money
• оставь 200 рублей чаевых = leave 200 rubles as a tip

Both can be used, with a nuance:

• на стол (accusative) emphasizes movement/destination: put it onto the table.
• на столе (prepositional) emphasizes location where it remains: leave it on the table (there). With оставить (“leave behind”), на столе is very common because the idea is “it stays there.”

Yes. Word order is flexible and changes emphasis:

• Оставь чаевые на столе. = neutral
• Чаевые оставь на столе. = emphasizes tips (as opposed to something else)
• На столе оставь чаевые. = emphasizes on the table (not somewhere else)