Дети сидели на лужайке и ели яблоки.

Дети means children. It is the irregular plural of ребёнок (child).

Russian has a number of nouns with irregular plural forms, and this is one of the most common:

• ребёнок = child
• дети = children

So a learner just has to memorize this pair.

Because both verbs are in the past tense plural.

In Russian, past tense verbs agree with the subject in gender/number:

• masculine singular: сидел
• feminine singular: сидела
• neuter singular: сидело
• plural: сидели

Since дети is plural, both verbs take the plural past ending:

• сидели = were sitting / sat
• ели = were eating / ate

Unlike English, Russian does not need an extra word like were here.

Russian does not have a special continuous tense like English was sitting or were eating.

Instead, Russian usually uses:

• past tense
• plus the imperfective aspect

So:

• сидели can mean were sitting
• ели can mean were eating

Depending on context, these can also sometimes translate more simply as sat and ate, but in this sentence the sense is naturally ongoing/background action: the children were sitting on the lawn and eating apples.

Because сидели comes from сидеть, which means to be sitting or to sit as a state.

Compare:

• сидеть → сидели = were sitting / sat
• сесть → сели = sat down

So:

• Дети сидели... = the children were sitting
• Дети сели... = the children sat down

English often uses sit for both ideas, but Russian clearly separates the state from the action of taking a seat.

This is an aspect question.

• есть → ели = to eat, imperfective
• съесть → съели = to eat up / finish eating, perfective

In this sentence, ели is used because the action is presented as an ongoing process, not as a completed result.

So:

• ели яблоки = they were eating apples
• съели яблоки = they ate the apples up / finished the apples

The sentence describes what the children were doing, so the imperfective is the natural choice.

Because на here means on in the sense of location, and that requires the prepositional case.

The noun is:

• лужайка = lawn, grassy patch

After на meaning on/at a place:

• лужайка → на лужайке

So:

• на лужайке = on the lawn

This is a very common pattern:

• на столе = on the table
• на улице = in the street / outside
• на лужайке = on the lawn

Лужайке is in the prepositional singular.

The preposition на can take different cases depending on meaning:

• location → prepositional
• motion onto something → accusative

So compare:

• на лужайке = on the lawn (location)
• на лужайку = onto the lawn (motion)

In your sentence, the children are already there, so Russian uses the prepositional:

• Дети сидели на лужайке = The children were sitting on the lawn.

Because яблоки is the accusative plural of яблоко (apple), and it is the direct object of ели.

• яблоко = apple
• яблоки = apples

With inanimate plural nouns, the accusative plural is usually the same as the nominative plural. So:

• nominative plural: яблоки
• accusative plural: яблоки

That is why Russian says:

• ели яблоки = ate apples / were eating apples

Because Russian normally has no articles.

English says:

• the children
• the lawn
• apples
• some apples

Russian often leaves all of that unstated:

• Дети
• на лужайке
• яблоки

Whether something is the, a, or some is usually understood from context.

So ели яблоки could mean:

• were eating apples
• were eating some apples
• were eating the apples

The exact English choice depends on context, not on a Russian article.

Yes. Russian word order is fairly flexible.

The sentence as given:

• Дети сидели на лужайке и ели яблоки.

This is a normal, neutral order.

But Russian could also say:

• На лужайке дети сидели и ели яблоки.
• Яблоки дети ели на лужайке.

These versions may shift the focus or emphasis, but the basic meaning remains similar because the case endings help show what each word is doing.

For a learner, the given version is the best neutral model.

A helpful stress pattern is:

Де́ти сиде́ли на лужа́йке и е́ли я́блоки.

Roughly:

• Де́ти = DYE-ti
• сиде́ли = si-DYE-li
• на лужа́йке = na lu-ZHAI-ke
• е́ли = YE-li
• я́блоки = YA-bla-ki

A few pronunciation notes:

• е at the start of a word often sounds like ye: е́ли
• Unstressed о is often reduced toward a
• Russian consonants can be soft, especially before е, и, я

If you can say it with the stresses in the right places, you will already sound much more natural.