Zatímco děti plavou u pláže, moje babička sedí pod stromem a čte knihu.

Zatímco means while. It connects two actions happening at the same time:

• Zatímco děti plavou... = While the children are swimming...
• ...moje babička sedí... = ...my grandmother is sitting...

It is similar to když, but not exactly the same.

• zatímco = while, often emphasizing simultaneity and sometimes contrast
• když = when, but in some contexts it can also mean while/as

In this sentence, zatímco is a very natural choice because two ongoing actions are happening at the same time.

Because Zatímco děti plavou u pláže is a subordinate clause, and Czech normally separates subordinate clauses from the main clause with a comma.

So the structure is:

• Zatímco děti plavou u pláže, = subordinate clause
• moje babička sedí pod stromem a čte knihu. = main clause

This is similar to English, where we also usually put a comma after an introductory while clause.

Czech does not have articles like English the and a/an.

So:

• děti can mean children, the children, or sometimes some children
• knihu can mean a book or the book
• stromem can mean a tree or the tree

The exact meaning depends on context. English speakers often look for articles, but Czech simply does not use them.

Yes. Děti is the plural of dítě (child), and it is irregular.

• singular: dítě = child
• plural: děti = children

In this sentence, děti is in the nominative plural because it is the subject of plavou.

This is a word you simply need to memorize, because it does not form its plural in the most predictable way.

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

Because the subject is děti (the children), the verb must match a plural subject:

• já plavu = I swim / am swimming
• ty plaveš = you swim
• on/ona plave = he/she swims
• oni/ony/ta děti plavou = they swim / are swimming

So děti plavou = the children are swimming.

Both are possible in Czech, but they mean slightly different things.

• u pláže = by the beach, near the beach
• na pláži = on the beach

So děti plavou u pláže suggests they are swimming near the beach or close to the shore, not literally on the beach.

Also, the preposition u always takes the genitive case, which is why pláž changes to pláže.

Because the preposition u requires the genitive case.

Base form:

• pláž = beach

Genitive singular:

• pláže

So:

• u pláže = by/near the beach

This is something learners need to get used to in Czech: many prepositions always require a certain case.

Because moje must agree with babička.

• babička is feminine singular
• the possessive my must match that noun

So:

• můj = masculine singular
• moje = feminine singular or neuter/plural form in many contexts

That is why moje babička means my grandmother.

You may also hear má babička, which also means my grandmother. Both are correct:

• moje babička
• má babička

Because Czech usually does not need subject pronouns when the subject is already clear.

The verb form tells you a lot, and in this sentence the noun is stated explicitly:

• moje babička sedí... = my grandmother is sitting...

Adding ona would usually be unnecessary here. Czech often omits pronouns that English must include.

So Czech prefers:

• Moje babička sedí pod stromem.

rather than:

• Moje babička ona sedí pod stromem. (unnatural here)

These are both 3rd person singular present tense forms, because the subject is moje babička (my grandmother), which is singular.

• sedět = to sit

• sedí = she sits / is sitting

• číst = to read
• čte = she reads / is reading

Czech present tense can often translate as either simple present or present continuous in English, depending on context:

• sedí = sits or is sitting
• čte = reads or is reading

Here, because of the context, English would normally say is sitting and is reading.

Because pod can take different cases depending on whether there is movement or location.

Here, the grandmother is already sitting under the tree, so this is a static location. That means pod takes the instrumental:

• pod stromem = under the tree

Compare:

• Sedí pod stromem. = She is sitting under the tree.
  
  • location, no movement
• Jde pod strom. = She is going under the tree.
  
  • movement toward a place

So in your sentence, stromem is instrumental singular.

Because knihu is the accusative singular of kniha (book).

• kniha = nominative, used for the subject
• knihu = accusative, used for the direct object

In this sentence, the book is what the grandmother is reading, so it is the direct object:

• čte knihu = she is reading a/the book

This is one of the most common case patterns in Czech:

• subject = nominative
• direct object = accusative

Because Czech does not have a separate grammatical form for the English present continuous.

English distinguishes:

• they swim
• they are swimming

Czech often uses the same present-tense form for both:

• plavou

So:

• děti plavou can mean the children swim or the children are swimming
• babička sedí can mean grandmother sits or is sitting
• čte knihu can mean reads a book or is reading a book

The context tells you it is an ongoing scene, so English naturally uses the continuous form.

Yes, Czech word order is more flexible than English, but the neutral order here is very natural.

This sentence has a clear structure:

• Zatímco děti plavou u pláže, moje babička sedí pod stromem a čte knihu.

You could change the order in some contexts, but the emphasis would change. Czech word order often reflects information structure, not just grammar.

For example, this sentence sounds neutral and descriptive. If you move parts around, it may sound more marked, poetic, or emphasize a different element.

So for a learner, this version is a good natural model to follow.