Dzieci biegają szybko po boisku.

Dzieci means children. It is the plural form of dziecko meaning child.

This is an irregular noun, so its plural is not formed in the most predictable way. You just need to learn:

• dziecko = child
• dzieci = children

In this sentence, dzieci is in the nominative plural, because it is the subject of the sentence — the ones doing the action.

Biegają is the 3rd person plural present tense form of biegać, which means to run.

It agrees with dzieci because children is plural.

A quick breakdown:

• ja biegam = I run / am running
• ty biegasz = you run / are running
• on/ona/ono biega = he/she/it runs / is running
• my biegamy = we run / are running
• wy biegacie = you (plural) run / are running
• oni/one biegają = they run / are running

So dzieci biegają = the children run / are running.

Polish often leaves out subject pronouns when they are not needed.

In English, you usually have to say they are running. In Polish, the verb ending already tells you who is doing the action:

• biegają already means they run / they are running

Since dzieci is already there as the subject, adding a pronoun would be unnecessary.

This is a very common question.

• biegać usually means to run around, to run repeatedly, or simply to be running in a general sense
• biec more often means to run in one specific instance, often with a clearer sense of direction

So:

• Dzieci biegają po boisku = The children are running around the field / run around the field
• Dzieci biegną do szkoły = The children are running to school

In your sentence, biegają fits very naturally because po boisku suggests movement around the field.

It can mean either, depending on context.

Polish present tense imperfective often covers both:

• They are running
• They run

So Dzieci biegają szybko po boisku could mean:

• The children are running quickly around the field
• The children run quickly around the field

Usually the situation tells you which meaning is intended.

Szybko means quickly or fast. It is an adverb, so it describes how the action is done.

Here it describes the verb biegają:

• biegają szybko = run quickly

It comes from the adjective szybki meaning fast. Compare:

• szybki chłopiec = a fast boy
• biega szybko = he runs quickly

Because the preposition po requires a different case here.

The basic dictionary form is:

• boisko = field / sports field / pitch

But after po in this meaning, Polish uses the locative case, so:

• po boisku = around the field / over the field

So you cannot keep boisko unchanged after po in this sentence.

Because it is in the locative singular.

The noun boisko is neuter, and after po it becomes:

• boisko → po boisku

This is a case ending change. In Polish, nouns often change their endings depending on their role in the sentence and which preposition is used.

So:

• boisko = nominative / dictionary form
• boisku = locative singular after po

Not exactly.

• po boisku suggests movement around, across, or all over the field
• na boisku means on the field or at the sports field

So:

• Dzieci biegają po boisku = The children are running around the field
• Dzieci są na boisku = The children are on the field

In your sentence, po boisku is used because the children are moving around within that space.

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

The neutral order here is:

• Dzieci biegają szybko po boisku.

But you could also hear:

• Dzieci szybko biegają po boisku.
• Szybko biegają dzieci po boisku.
• Po boisku dzieci biegają szybko.

These versions all keep roughly the same basic meaning, but the emphasis changes.

For example:

• Dzieci szybko biegają po boisku puts a little more focus on quickly
• Po boisku dzieci biegają szybko gives more prominence to around the field

A rough pronunciation guide is:

JYE-chee byeh-GAI-om SHIP-koh poh boh-EE-skoo

A few helpful points:

• dz in dzieci is part of a soft sound
• ci in dzieci sounds a bit like chee
• bie in biegają sounds roughly like byeh
• ą in biegają is a nasal vowel; at the end of the word it is often heard somewhat like om
• sz in szybko sounds like sh
• boisku has three clear vowel parts: bo-i-sku

You do not need to pronounce it exactly like the English guide above, but it can help you get close.

Here dzieci is in the nominative plural, because it is the subject of the sentence.

It answers the question who is running?

• Dzieci are running.

So the sentence structure is:

• Dzieci = subject
• biegają = verb
• szybko = adverb
• po boisku = prepositional phrase showing where the action happens

Yes, it is completely natural.

Dzieci biegają szybko po boisku. sounds like normal everyday Polish and is a good example of:

• a plural subject
• a present-tense verb
• an adverb
• a prepositional phrase with the locative

It is the kind of sentence you could easily hear in a real situation, especially when talking about children at school, sports practice, or play.