На уроке дети сначала сидели поодиночке, а потом сообща сделали большой плакат.

In natural English, На уроке means in class, during the lesson, or at the lesson.

Russian often uses на with events or organized activities:

• на уроке = in class / during the lesson
• на лекции = at the lecture
• на работе = at work
• на концерте = at the concert

So although на often means on, here you should think of it as at/during.

Because на here requires the prepositional case when it means location or setting.

• урок = dictionary form, nominative
• на уроке = prepositional singular

So:

• урок = lesson
• на уроке = in class / during the lesson

This is a very common pattern:

• в школе = at school
• на столе = on the table
• в комнате = in the room

This is a very common way to show sequence:

• сначала = at first / first
• а потом = and then / but then / afterwards

In this sentence, it marks two stages:

1. first the children were sitting separately
2. then they made a big poster together

The а here is not a strong but. It often links two contrasting stages or situations:

• Сначала было тихо, а потом стало шумно. = At first it was quiet, and then it got noisy.

Because сидели describes a state or ongoing situation: they were sitting.

• сидеть = to sit, to be sitting
• сесть = to sit down

So:

• дети сидели = the children were sitting
• дети сели = the children sat down

Here the point is not the moment they took their seats, but the fact that for some time they were seated separately.

Поодиночке means one by one, separately, or individually.

In this sentence, сидели поодиночке means the children were sitting separately, not working as a group yet.

It is an adverb, so it describes how they were sitting.

Compare:

• поодиночке = separately, individually
• вместе = together

So the sentence contrasts:

• first: поодиночке
• later: сообща

Сообща means jointly, collectively, together as a group.

It is close to вместе, but it often emphasizes cooperation or shared effort a bit more.

So:

• вместе = together
• сообща = together, jointly, by working together

In this sentence, сообща сделали большой плакат suggests that the children did not just happen to be in the same place; they worked together as a group to make the poster.

Because сделали is perfective, and here the sentence focuses on the completed result: they ended up making a big poster.

• делали = were making / used to make / worked on making
• сделали = made, completed, finished making

The sentence presents a clear outcome:

• first they sat separately
• then they successfully produced a poster

If you said делали, it would sound more like describing the process, without necessarily emphasizing completion.

It is in the accusative, but for an inanimate masculine noun, the accusative looks exactly like the nominative.

• nominative: большой плакат
• accusative: большой плакат

Since плакат is masculine and inanimate, nothing changes in form here.

Compare with an animate masculine noun:

• Я вижу большого мальчика. = I see a big boy.

But with an inanimate noun:

• Я вижу большой плакат. = I see a big poster.

Дети is the normal plural form meaning children. It is suppletive, which means it does not look like a regular plural of ребёнок.

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

This is similar to English person / people or child / children: the plural is irregular and must simply be learned.

Yes, Russian word order is fairly flexible, but the original order is natural and clear.

Original:

• На уроке дети сначала сидели поодиночке, а потом сообща сделали большой плакат.

This order works well because it presents:

1. the setting: На уроке
2. the subject: дети
3. the timeline: сначала ... а потом ...

You could move things around for emphasis, but the neutral version is the one you have.

For example:

• Дети на уроке сначала сидели поодиночке, а потом сообща сделали большой плакат.

This is also possible, but it slightly shifts the emphasis onto дети.

Usually yes. Плакат most often means poster.

In a school context, it can also suggest a large illustrated sheet, display poster, or project poster made together for class. So depending on context, English translations might be:

• poster
• big poster
• large class poster
• display board sometimes, if that fits the situation better

But poster is the standard basic meaning.

Because Russian usually does not need subject pronouns when the verb form already shows who is doing the action.

• сделали = they made
• сидели = they were sitting

Since дети has already been mentioned, repeating они would usually be unnecessary unless you wanted special emphasis or contrast.

So Russian often sounds more natural with no explicit they:

• дети ... сидели ..., а потом сделали ...

English needs the subject more often; Russian often does not.