O přestávce si děti hrají na hřišti.

O přestávce means during recess / at break time.

Grammatically, přestávce is in the locative singular. The preposition o normally takes the locative in expressions like this.

This is a very common Czech time expression pattern:

• o víkendu = at the weekend
• o prázdninách = during the holidays
• o přestávce = during recess

The dictionary form is přestávka. After o, it must go into the locative singular, so it becomes přestávce.

Two things happen:

• the ending changes from -a to -e
• the k changes to c

So:

• přestávka → o přestávce

This kind of change is normal in Czech declension.

Here si is part of the verb expression hrát si, which means to play in the sense of amuse oneself / play around, especially for children.

So:

• hrát si = to play
• hrát without si often means things like play a game, play an instrument, or play a role

Examples:

• Děti si hrají. = The children are playing.
• Hrají fotbal. = They are playing football.
• Hraje na klavír. = He/She plays the piano.

In this sentence, si does not literally mean to themselves in a natural English way. It is just part of how Czech says play here.

Because si is a clitic: a short unstressed word that usually goes in second position in the clause.

In O přestávce si děti hrají na hřišti, the first unit is O přestávce, so si comes right after it.

Compare:

• Děti si hrají na hřišti.
• O přestávce si děti hrají na hřišti.

This is a very important Czech word-order rule. Words like si, se, mi, ti, jsem, bych often behave this way.

You can, but it changes the feel of the sentence.

• Děti si hrají na hřišti = The children are playing on the playground.
  This is the natural way to describe children at play.

• Děti hrají na hřišti = The children play / are playing on the playground.
  This can sound less complete by itself, and often makes you expect what they are playing, for example fotbal.

So in this sentence, si is the most natural choice.

Hrají is the 3rd person plural present tense form of hrát.

The subject is děti = children, which is plural, so the verb must also be plural:

• dítě si hraje = a child is playing
• děti si hrají = children are playing

So the ending -í here marks they.

Yes. The singular is dítě (child), and the plural is děti (children), so it is not formed in a simple regular way.

Useful forms:

• dítě = child
• děti = children
• dětí = of children

So this is a noun worth memorizing separately.

Na hřišti means on the playground / at the playground.

Here na expresses location, so it takes the locative case:

• na hřišti = on/at the playground

The dictionary form is hřiště, and its locative singular is hřišti:

• hřiště → na hřišti

Also compare location vs motion:

• na hřišti = on the playground, at the playground
• na hřiště = onto / to the playground

That difference is very common with Czech prepositions.

Because Czech normally uses na with hřiště.

This is something you largely have to learn as part of the noun/preposition combination. Czech often uses na with open areas, surfaces, or certain public places:

• na hřišti = on the playground
• na zahradě = in the garden
• na stadionu = at the stadium

So even though English often says in or at, Czech idiomatically says na hřišti.

No. That is a very good thing to notice.

There are two different patterns:

1. hrát si na + accusative
   = pretend to be

   • hrát si na doktora = to play/pretend to be a doctor

2. na + locative
   = location

   • na hřišti = on the playground

In your sentence, na hřišti is just a place expression. It is not the pretend to be pattern.

A useful clue is the case:

• na doktora = accusative
• na hřišti = locative

Yes. Czech word order is flexible, and changing it usually changes emphasis, not the core meaning.

For example:

• O přestávce si děti hrají na hřišti.
• Děti si o přestávce hrají na hřišti.
• Na hřišti si děti hrají o přestávce.

These all mean roughly the same thing, but the first one emphasizes when, the last one emphasizes where.

One important thing stays true: si still tends to stay in second position.

Because Czech does not have articles like English a and the.

So Czech simply says:

• děti = children / the children
• hřiště = a playground / the playground

Which meaning is intended depends on context. English has to choose an article, but Czech usually does not.

A few useful pronunciation points:

• Czech stress is usually on the first syllable of each word:

  • O PŘE-stáv-ce si DĚ-ti HRA-jí na HŘIŠ-ti

• c is pronounced like ts

  • so přestávce ends with -tse

• š is like English sh

  • so hřišti contains sh

• ř is the famous Czech sound with no exact English equivalent.
  It is roughly like an r plus a zh sound together.
  You do not need to master it immediately to be understood.

If you want a rough learner-friendly approximation:

• přestávce ≈ pshes-taav-tse
• hřišti ≈ hrzhish-ti

Not perfect, but enough to help you get started.