На тарілці лежать помідор, огірок і цибуля.

Because after на meaning location, Ukrainian uses the locative case.

• dictionary form: тарілка
• after на for location: на тарілці

So на тарілці means on the plate.

There is also a stem change here:

• к often changes to ц before -і
• so тарілка → тарілці

This is a very common pattern in Ukrainian nouns.

Тарілці is in the locative singular.

In this sentence, на means on in the sense of where something is located, so it takes the locative:

• на столі = on the table
• на підлозі = on the floor
• на тарілці = on the plate

If на showed motion toward something, it would usually take the accusative instead, but that is not what is happening here.

Лежать is the plural form of лежати, which means to lie or to be lying.

In this sentence, it is used naturally for food items resting on a plate. Ukrainian often uses position verbs where English might just say is/are.

So the idea is not just existence, but also physical position: the items are lying there on the plate.

Compare:

• лежати = lie
• стояти = stand
• висіти = hang

Because the subject is actually a list of three things:

• помідор
• огірок
• цибуля

Together, they form a compound subject, so Ukrainian uses a plural verb:

• лежать = lie / are lying

Even though each noun is singular, the whole subject is plural in meaning: there are three separate items.

Лежить is singular, so it would be used with just one item:

• На тарілці лежить помідор. = A tomato is lying on the plate.

But with several items joined by і, Ukrainian uses the plural:

• На тарілці лежать помідор, огірок і цибуля.

So лежить would not match this subject.

Because they are the subject of the sentence, not the object.

The vegetables are the things that are lying on the plate, so they stay in the nominative case:

• помідор
• огірок
• цибуля

If they were direct objects, then you might see accusative forms instead.

Because цибуля is nominative singular, and here it is part of the subject.

• цибуля = nominative
• цибулю = accusative

Since the onion is one of the things doing the action of lying on the plate, nominative is correct.

A quick contrast:

• На тарілці лежить цибуля. = An onion is lying on the plate.
• Я бачу цибулю. = I see an onion.

Yes, you can, but the nuance is a little different.

• лежать focuses on the items’ physical position
• є focuses more on existence / presence

So:

• На тарілці лежать... = they are lying on the plate
• На тарілці є... = there is / there are ... on the plate

For objects like vegetables placed on a plate, лежать sounds very natural.

Because Ukrainian does not have articles like English a/an/the.

So помідор can mean:

• a tomato
• the tomato

The exact meaning depends on context.

That is why a short Ukrainian sentence can look more bare than its English equivalent.

Ukrainian word order is fairly flexible, and speakers often put the location or context first.

Starting with На тарілці does this:

• sets the scene first
• tells you where to imagine the objects
• sounds natural in description

You could also say:

• Помідор, огірок і цибуля лежать на тарілці.

That is also correct, but it gives a slightly different emphasis. The original sentence highlights the plate/location first.

Yes, Ukrainian has several common ways to say and:

• і
• й
• та

In many cases they mean basically the same thing, but the choice is often made for sound and smoothness.

• і is the most basic and neutral form
• й is often used to avoid awkward sound combinations
• та is also common and natural in many contexts

In this sentence, і is completely standard:

• помідор, огірок і цибуля

It can mean either, depending on context.

In this sentence, it most naturally means an onion as one item on the plate, because it is listed together with помідор and огірок.

So although цибуля can sometimes behave like a mass noun in English translation, here it is understood as a single onion.

Yes. It sounds natural and grammatical.

A Ukrainian speaker would easily understand it as a simple description of what is on the plate. The sentence uses:

• a normal location phrase: на тарілці
• a natural position verb: лежать
• a straightforward list of nouns in the nominative

So it is a good model sentence for basic descriptive Ukrainian.