U řeky je malá pláž, kde si děti hrají a lidé tam v létě plavou.

U řeky means by the river or near the river.

The preposition u takes the genitive case, so řeka changes to řeky.

• dictionary form: řeka = river
• after u: u řeky = by the river

This is a very common pattern in Czech:

• u domu = by the house
• u školy = by the school
• u lesa = by the forest

So řeky is genitive singular because of u.

Both are possible in Czech, but U řeky je malá pláž is the most natural here.

Czech word order is flexible, but it often puts:

• known setting or location first
• then the new information

So the sentence structure is roughly:

• U řeky = setting
• je malá pláž = new information about that setting

This sounds natural because it introduces the place first, then tells you what is there.

Compare:

• U řeky je malá pláž. = By the river, there is a small beach.
• Malá pláž je u řeky. = The small beach is by the river.

The second version is grammatical too, but the emphasis is different.

The adjective malá agrees with the noun pláž.

Pláž is:

• feminine
• singular
• nominative here

So the adjective must also be:

• feminine
• singular
• nominative

That gives malá.

Agreement is a basic Czech rule:

• malý dům = small house
• malá pláž = small beach
• malé město = small town

So the ending -á shows feminine singular nominative.

Kde means where in the sense of in which place.

In this sentence, kde introduces a relative clause that describes pláž:

• malá pláž, kde si děti hrají...
• a small beach where children play...

So kde connects the second part of the sentence to pláž.

It works like English where:

• dům, kde bydlím = the house where I live
• město, kde pracuje = the city where he/she works

Here it refers back to pláž.

In hrát si, the si is part of the verb expression and usually means to play in the sense of children playing, having fun, amusing themselves.

So:

• hrát can mean to play something, like a game or an instrument
• hrát si means to play around / to be playing / to amuse oneself

Examples:

• Děti si hrají. = The children are playing.
• Hraju fotbal. = I play football.
• Hraju na klavír. = I play the piano.

So in this sentence, si is not optional. Děti hrají by itself would usually sound incomplete or mean they are playing something.

Yes, hrají is present tense, from hrát si.

It is used because the sentence describes a general scene or habitual activity:

• there is a beach there
• children play there
• people swim there in summer

Czech present tense can express:

• what is happening now
• what usually happens
• a general description

Here it is more like a general description of the place, not necessarily only this exact moment.

Děti is the nominative plural form of dítě = child.

Because děti is the subject of hrají, it must be in the nominative case.

This noun is irregular, so it is worth memorizing:

• singular: dítě
• plural: děti

Examples:

• Dítě spí. = The child is sleeping.
• Děti si hrají. = The children are playing.

Good question. Tam means there, and it is not strictly necessary for basic grammar. The sentence would still work without it:

• ...kde si děti hrají a lidé v létě plavou.

But tam is very natural in Czech because it reinforces the location:

• children play there
• people swim there in summer

So it adds a sense of in that place. Czech often uses words like tam even when English might leave them out.

It makes the sentence sound a bit more flowing and natural.

V létě means in summer.

The preposition v often takes the locative case when it means in a place or time period. Here it is time, so léto changes to létě.

• dictionary form: léto = summer
• after v: v létě = in summer

Other similar examples:

• v zimě = in winter
• na jaře = in spring
• na podzim = in autumn

So létě is locative singular.

Both forms exist, but they are a little different in style.

• lidé is the standard nominative plural form of člověk in the meaning people
• lidi is very common in everyday speech, but it is more colloquial and often used in non-nominative contexts too

In a neutral written sentence like this, lidé is a very natural choice.

So:

• Lidé tam plavou. = standard
• Lidi tam plavou. = common in speech, more informal

Plavou is the 3rd person plural present tense of plavat = to swim.

Because the subject is lidé = people, the verb must be plural:

• člověk plave = a person swims
• lidé plavou = people swim

This is an imperfective verb, which fits well because the sentence describes a usual or repeated activity.

Yes. Czech word order is flexible, and changing it changes the emphasis.

The version in the sentence is very natural:

• lidé tam v létě plavou

Other possible versions:

• lidé v létě tam plavou
• tam lidé v létě plavou
• v létě tam lidé plavou

These are all understandable, but they do not sound equally neutral in every context.

The original order is good because it flows naturally:

• subject: lidé
• place reminder: tam
• time: v létě
• verb: plavou

The sound ř is one of the most famous difficult Czech sounds.

It is a raised, vibrating sound somewhat between:

• r
• zh as in measure

For many learners, the best first step is not to try to make it perfect immediately. A useful approximation is:

• start from a rolled or tapped r
• add a zh-like quality

So řeky is approximately like rzheh-ki, but that is only a rough guide.

A few pronunciation notes from the sentence:

• řeky = first syllable stress: ŘE-ky
• děti sounds roughly like dyeh-ti
• hrají has a clear h and long final vowel sequence
• plavou sounds roughly pla-vou

If you cannot say ř yet, a plain r is often understood, even though it is not fully correct.

It most naturally describes a general or typical situation.

The present tense here paints a scene:

• there is a small beach by the river
• children play there
• people swim there in summer

So it sounds like a description of the place and what people usually do there, not just one exact moment.

Czech often uses the present tense this way, just like English can in descriptive sentences.