Copiii se joacă pe teren.

Because copiii is the children:

• copil = child
• copii = children
• copiii = the children

The extra i comes from the Romanian definite article attached to the plural form. So:

• plural noun: copii
• plural + definite article: copiii

This is a very common thing in Romanian, even though it looks strange at first to English speakers.

Here, se is part of the verb a se juca, which means to play in the sense of to play around / have fun.

So:

• se joacă = play / are playing

In this sentence, you should learn a se juca as a whole verb. The se is not optional here.

This is an important distinction.

• a se juca = to play, have fun, play around
• a juca = to play something, or sometimes to perform/act, depending on context

For example:

• Copiii se joacă. = The children are playing.
• Copiii joacă fotbal. = The children play football.

So in Copiii se joacă pe teren, the meaning is general: they are playing there, not necessarily naming a specific game.

In Romanian present tense, the 3rd person singular and 3rd person plural often have the same form.

So:

• el/ea se joacă = he/she plays
• ei/ele se joacă = they play

That is completely normal. Romanian often relies on the subject noun or pronoun to show whether it is singular or plural.

It is the present tense.

Copiii se joacă pe teren can mean:

• The children play on the field
• The children are playing on the field

Romanian does not usually make the same strong distinction English makes between play and are playing. The exact meaning depends on context.

Pe teren literally means on the field / on the ground / on the court, depending on context.

The noun teren is broad and can refer to things like:

• a sports field
• a court
• a pitch
• a playground area
• a piece of ground/land

So the most natural English translation depends on the situation.

Romanian often uses location expressions without the definite article where English might use the.

So pe teren can naturally mean:

• on the field
• on the court
• out on the field

If you want to make it more specifically identified, Romanian can use the definite form, especially when something else follows:

• pe terenul școlii = on the school’s field
• pe terenul de sport = on the sports field/court

So pe teren is perfectly natural here.

The spelling looks harder than it really is.

A useful learner-friendly way to think of it is:

• copii ≈ ko-PEE
• copiii ≈ ko-PEE(y) or a slightly longer ee sound with a very light final y-like sound

The important thing is not to panic when you see three i letters. They reflect grammar more than a dramatically different pronunciation.

It is roughly:

• JOA-că

A few pronunciation notes:

• joa- has a diphthong, something like joa
• ă is the Romanian schwa sound, like the a in sofa
• the stress is on the first syllable: JOA-că

So it is not pronounced like English joke or jog.

Yes. Romanian word order is more flexible than English word order.

The neutral order here is:

• Copiii se joacă pe teren.

But you could also say:

• Pe teren, copiii se joacă.

That shifts the focus toward pe teren.

Even so, the original sentence is the most straightforward, neutral way to say it.

The singular version is:

• Copilul se joacă pe teren. = The child is playing on the field.

Related forms:

• un copil = a child
• copilul = the child
• copii = children
• copiii = the children

This is a good set to memorize together.

Not if you want the same meaning.

• Copiii se joacă pe teren = The children are playing on the field.
• Copiii joacă pe teren sounds incomplete or changes the meaning, because a juca usually wants a clearer object or activity, such as fotbal.

For example:

• Copiii joacă fotbal pe teren. = The children are playing football on the field.

So for general playing around, keep se.