На полке стоят пшено и перловка, но дети почему-то выбирают макароны.

Because together they form a compound subject: пшено и перловка = millet and pearl barley.

In Russian, when two things are joined by и (and), the verb is normally plural:

• Пшено и перловка стоят... = Millet and pearl barley are standing...
• Compare: Пшено стоит... = The millet is standing...
• Перловка стоит... = The pearl barley is standing...

So стоят is plural because the sentence is talking about two items together.

Russian often uses position verbs where English simply says is/are.

Common ones are:

• стоять = to stand
• лежать = to lie
• висеть = to hang

So На полке стоят... literally means On the shelf, ... are standing.

This sounds natural in Russian if the items are thought of as being in packages, jars, containers, or just placed upright on a shelf. English usually avoids this and simply says There is/There are ... on the shelf.

Russian is often more physically specific than English in this kind of sentence.

Yes, you could, but the meaning and feel are slightly different.

• На полке есть пшено и перловка = There is millet and pearl barley on the shelf.
  This focuses on existence/presence.
• На полке стоят пшено и перловка = Millet and pearl barley are standing on the shelf.
  This gives a more visual/spatial picture.

So есть is more neutral, while стоят is more descriptive.

Because на with location usually takes the prepositional case.

• полка = shelf
• на полке = on the shelf

This is the normal pattern when you mean where something is:

• на столе = on the table
• на стене = on the wall
• на полке = on the shelf

If you were talking about movement onto the shelf, you would use the accusative:

• положить на полку = to put onto the shelf

So:

• на полке = location
• на полку = destination

Почему-то means for some reason.

The particle -то adds a sense like:

• somehow
• for some reason
• in some unspecified way

So:

• дети почему-то выбирают макароны = the children, for some reason, choose pasta

The hyphen is standard Russian spelling for this kind of particle:

• кто-то = someone
• что-то = something
• где-то = somewhere
• почему-то = for some reason

It often suggests that the speaker finds it a little surprising or unexplained.

Because макароны is one of those Russian nouns that is normally used only in the plural.

This is similar to English words like:

• scissors
• pants

Even though in English we often say pasta as an uncountable noun, Russian commonly says макароны as a plural noun.

So:

• макароны вкусные = the pasta is tasty
• выбирают макароны = they choose pasta

Russian treats it grammatically as plural, even when the meaning in English may be more mass/uncountable.

Because it is accusative plural, and for inanimate plural nouns, the accusative is usually the same as the nominative.

So:

• nominative: макароны
• accusative: макароны

That is why you see:

• макароны лежат на столе = pasta is on the table
• дети выбирают макароны = the children choose pasta

Same form, different function.

This is a very common Russian pattern:

• я вижу столы = I see tables
• они покупают яблоки = they buy apples

For inanimate plurals, nominative and accusative often match.

Дети is in the nominative plural because it is the subject of выбирают.

• дети = children
• выбирают = choose

So the structure is:

• дети выбирают макароны = the children choose pasta

The subject of a normal finite verb is usually in the nominative case.

Also note that дети is an irregular plural form of ребёнок:

• singular: ребёнок = child
• plural: дети = children

Both are possible.

Russian word order is more flexible than English, and changes in word order often affect focus rather than basic meaning.

• На полке стоят пшено и перловка
  Focuses first on the location: on the shelf...
• Пшено и перловка стоят на полке
  Focuses first on the items: millet and pearl barley...

In this sentence, starting with На полке sounds natural because it sets the scene before contrasting it with what the children choose.

Но means but and gives a stronger sense of contrast.

Here the idea is:

• these grains are available,
• but the children choose pasta instead.

So но fits well.

You might sometimes see а in Russian for contrast too, but а often feels more like whereas / while / and on the other hand, not necessarily a strong contradiction.

• но = but
• а = and/whereas/but in a softer contrastive sense

In this sentence, но is the more natural choice because the speaker seems to mean something like however, despite that...

Russian has no articles.

So nouns can mean:

• a shelf
• the shelf

depending on context.

In this sentence:

• На полке can mean on a shelf or on the shelf
• дети can mean children or the children
• макароны can mean pasta or the pasta

English forces you to choose an article, but Russian usually leaves that to context.

Yes. They are both food words, but they belong to different noun patterns.

• пшено = millet groats; neuter
• перловка = pearl barley; feminine

Their genders come from their endings:

• -о often suggests neuter → пшено
• -а often suggests feminine → перловка

This matters for agreement in the singular:

• вкусное пшено
• вкусная перловка

But in your sentence, since they are joined together with и, the verb becomes plural: стоят.

Выбирают is imperfective, from выбирать.

The imperfective is used here because the sentence describes a general tendency or repeated behavior:

• the children choose pasta
• they tend to pick pasta

If you used the perfective выберут, it would usually point to a single completed choice in the future:

• Дети выберут макароны = The children will choose pasta

So выбирают is right for a general present-time statement.