Когда вода тёплая, дети любят плескаться у берега.

Because Когда вода тёплая is a subordinate clause, and Russian normally separates subordinate clauses from the main clause with a comma.

So the structure is:

• Когда вода тёплая, — When the water is warm
• дети любят плескаться у берега. — children like to splash near the shore.

Even when the subordinate clause comes first, the comma is still required.

Because тёплая must agree with вода.

The noun вода is:

• feminine
• singular
• nominative

So the adjective also has to be:

• feminine
• singular
• nominative

That gives тёплая.

Compare:

• тёплый чай — warm tea
• тёплая вода — warm water
• тёплое молоко — warm milk

In the present tense, Russian usually leaves out the verb to be.

So:

• вода тёплая literally looks like water warm
• but it means the water is warm

This is completely normal in Russian.

In past and future tenses, forms of to be do appear:

• Вода была тёплая. — The water was warm.
• Вода будет тёплая. — The water will be warm.

Дети is the common plural form meaning children. It is actually the plural of ребёнок.

This is an irregular pattern:

• ребёнок — child
• дети — children

Russian has a few irregular plurals like this, and this is one of the most common ones.

Любят is the 3rd person plural form of любить — to love / to like.

The subject is дети — children, which is plural, so the verb must also be plural:

• я люблю — I like
• ты любишь — you like
• он/она любит — he/she likes
• они любят — they like

Since дети = they, you get дети любят — children like.

Плескаться means to splash around, to frolic in the water, or to paddle/splash about.

The ending -ся often makes a verb reflexive, but in many verbs it creates a meaning that is best learned as a whole word rather than translated mechanically.

Here:

• плескать can mean to splash something
• плескаться means to splash around oneself, to splash about

So дети любят плескаться means the children like splashing around.

After любить meaning to like/love doing something, Russian commonly uses the infinitive.

So the pattern is:

• любить + infinitive

Examples:

• Я люблю читать. — I like to read.
• Они любят гулять. — They like to walk.
• Дети любят плескаться. — Children like to splash around.

This is similar to English like to do something.

Because the preposition у requires the genitive case.

The noun is:

• dictionary form: берег — shore, bank

After у, it changes to genitive singular:

• у берега

So:

• у берега = by the shore, near the shore

This is a very common pattern:

• у дома — by the house
• у моря — by the sea
• у реки — by the river

Not exactly. У usually means by, near, or close to.

So у берега suggests being near the shore rather than literally on top of it.

Depending on context, it can be translated as:

• by the shore
• near the shore
• close to the shore

That is why it fits well with children splashing in shallow water.

Yes. Russian word order is fairly flexible, though the original version is the most neutral and natural.

Original:

• Когда вода тёплая, дети любят плескаться у берега.

You could also say:

• Дети любят плескаться у берега, когда вода тёплая.

The meaning stays basically the same, but the focus changes slightly:

• starting with Когда вода тёплая emphasizes the condition first
• starting with Дети любят... emphasizes the children’s action first

Yes. In Russian, ё is often replaced in writing by е, especially in everyday texts.

So you may see:

• тёплая
• теплая

They mean the same thing, but тёплая shows the correct pronunciation more clearly.

For learners, it is helpful to remember that ё is stressed:

• тЁплая

The stress is on the first syllable:

• плЕскаться

A rough pronunciation guide:

• PLYES-ka-tsa

A few notes:

• е after пл sounds like ye
• -ться / -тся in verbs is pronounced roughly like -ца
• the written ь is not pronounced as a separate sound

So плескаться sounds closer to PLYES-ka-tsa than to a fully letter-by-letter pronunciation.