Мы с подругой долго наблюдали, как дети играют на площадке.

Because the preposition с meaning with requires the instrumental case.

• подруга = nominative
• с подругой = instrumental

So Мы с подругой literally means we with a female friend / my female friend and I, but in natural English it is usually My friend and I.

Usually here it means female friend, not necessarily a romantic partner.

In Russian:

• друг = male friend
• подруга = female friend

Depending on context, подруга can sometimes be understood as a girlfriend in the romantic sense, but in a simple sentence like this, learners should first read it as female friend.

Because мы с подругой functions like my friend and I.

Russian often uses the pattern:

• мы с братом = my brother and I
• мы с мамой = my mom and I
• мы с подругой = my female friend and I

If you said я с подругой, that would mean I am with my friend, but it would not itself mean my friend and I as the subject of the action in the same natural way.

Because the subject is effectively two people: мы + подруга.

Even though the phrase is structured as мы с подругой, it still refers to a plural subject, so the verb must be plural:

• мы наблюдали = we watched
• мы с подругой наблюдали = my friend and I watched

The imperfective is used because the action is presented as a process that lasted for some time.

That fits especially well with долго:

• долго наблюдали = watched for a long time

If Russian used a perfective form here, the feeling would be more about a completed act, not the ongoing activity of watching. The imperfective is the natural choice when you want to emphasize duration.

Good question. Russian often uses наблюдать за + instrumental when the object is a noun:

• наблюдать за детьми = to watch the children
• наблюдать за птицами = to observe birds

But here the object is not just a noun. It is a whole clause:

• как дети играют на площадке

So наблюдали, как... is a normal pattern meaning watched how / watched as...

Both structures are possible in Russian, but they are built differently:

• наблюдали за детьми = watched the children
• наблюдали, как дети играют = watched the children play / watched as the children were playing

Because как дети играют на площадке is a subordinate clause.

Russian uses commas very regularly to separate main clauses from subordinate clauses. So the comma here is required:

• Мы с подругой долго наблюдали, как дети играют на площадке.

In English, punctuation works differently, but in Russian this comma is standard.

Russian normally uses a full clause after verbs like watch, see, hear, and notice in this kind of sentence.

So instead of copying the English pattern watched the children play, Russian often says:

• наблюдали, как дети играют
• literally: watched, how the children are playing

This is one of those places where Russian prefers a structure different from English. It is very natural in Russian.

Because Russian can use the present tense in a как-clause after a past verb of perception to show the action as unfolding before someone’s eyes.

So:

• наблюдали, как дети играют feels like watched the children playing / watched as the children were playing

This does not mean the children are still playing now. It is just a common Russian way to present the scene vividly.

You could also hear как дети играли, which is also possible, but играют makes the action feel more immediate and live.

Долго is an adverb, and it modifies наблюдали.

It tells you for how long the action happened:

• долго наблюдали = watched for a long time

It does not describe the children’s playing. It describes the duration of the watching.

It is the prepositional case after на.

• площадка = nominative
• на площадке = on/at the playground

In Russian, на is often used for open areas, surfaces, or certain public spaces. A playground is naturally expressed as на площадке, not usually в площадке.

So:

• на площадке = on the playground / at the playground

No, Russian word order is fairly flexible, but this order is natural and neutral.

The sentence starts with the subject:

• Мы с подругой

then the adverb and verb:

• долго наблюдали

then the subordinate clause:

• как дети играют на площадке

You could move some parts around for emphasis, but the given version sounds smooth and standard. For example, putting долго before мы or moving на площадке earlier would change the focus slightly, even if the basic meaning stayed the same.