Лучше вернуться домой засветло, потому что затемно у реки становится холоднее.

In Russian, лучше + infinitive is a very common way to give general advice or make a recommendation.

So Лучше вернуться домой засветло means something like:

• It’s better to return home before dark
• You’d better get home while it’s still light

Russian does not need a dummy subject like it. If you wanted to name the person more explicitly, you could say Тебе лучше вернуться домой засветло.

Вернуться is perfective, so it presents the return as one whole completed action: getting back home.

That fits this sentence, because the speaker is advising someone to complete the return before dark.

Compare:

• вернуться = to return, to get back once, successfully
• возвращаться = to be returning / to return regularly / to return in a general or ongoing sense

So here вернуться is the natural choice.

Here -ся is part of the verb itself. Вернуться means to return / come back.

This is different from вернуть, which means to return something / give something back.

So:

• вернуть книгу = to return a book
• вернуться домой = to return home

For a learner, it is best to treat вернуться as a separate dictionary verb, not as a passive form.

Домой is a very common adverb meaning homeward / to home / home when talking about movement toward home.

Compare:

• домой = to home
• дома = at home
• дом = house/home as a noun

So:

• Иду домой = I’m going home
• Я дома = I’m at home

In your sentence, movement is involved, so домой is exactly right.

These are fixed adverbs connected with light and darkness.

• засветло = while it is still light, before it gets dark
• затемно = in the dark, when it is dark

In this sentence, they create a neat contrast:

• return засветло
• because затемно it gets colder by the river

A useful warning: затемно can sometimes mean before dawn, while it is still dark in other contexts, such as встать затемно. Here, because it is contrasted with засветло, it is most naturally understood as once it’s dark / in darkness.

Because the preposition у takes the genitive case.

река is the dictionary form. In the genitive singular it becomes реки.

So:

• река = river
• у реки = by the river / near the river

This is a very common pattern:

• у дома = by the house
• у окна = by the window
• у реки = by the river

This is an impersonal construction. Russian often talks about weather, temperature, and environmental conditions without any subject.

English needs it:

• It gets colder by the river

Russian does not:

• У реки становится холоднее

Literally, it is something like By the river, becomes colder. This is completely normal Russian.

Холоднее is the comparative form, meaning colder.

It comes from холодный / холодно. In Russian, the comparative form is often the same whether English would use an adjective or an adverb.

So here:

• холодный = cold
• холоднее = colder

In становится холоднее, it means becomes colder.

Because потому что introduces a subordinate clause: the reason or explanation.

Main clause:

• Лучше вернуться домой засветло

Reason clause:

• потому что затемно у реки становится холоднее

Russian normally puts a comma before потому что, just as English separates a because-clause in many similar sentences.

No, Russian word order is fairly flexible, but the version you have sounds natural and balanced.

Current order:

• Лучше вернуться домой засветло — first the advice, then the time condition
• потому что затемно у реки становится холоднее — first the reason, with затемно placed early for emphasis

If you changed the order, the basic meaning would stay similar, but the focus would shift slightly. Russian often uses word order to highlight what feels most important or most natural in context, rather than following a rigid pattern like English.

Sometimes yes, but not always exactly.

Лучше вернуться домой засветло can mean:

• It’s better to return home before dark
• You’d better return home before dark

The English had better often sounds a bit stronger, more like a warning. Russian лучше + infinitive can be neutral advice or a stronger recommendation depending on context.

In this sentence, because of the explanation about the cold, you’d better is a very reasonable translation.