Надень шарф потеплее: вечером у реки будет ветрено.

Because in standard Russian, надеть means to put on (something on yourself), while одеть means to dress someone else.

So:

• надеть шарф = to put on a scarf
• одеть ребёнка = to dress a child

Here the speaker is telling someone to put on a scarf, so Надень is correct.

Also, Надень is the imperative singular form of надеть, so it means Put on.

Шарф is in the accusative singular, because it is the direct object of Надень.

The dictionary form is шарф. Since it is a masculine inanimate noun, its accusative singular looks exactly like the nominative singular:

• nominative: шарф
• accusative: шарф

So even though the form does not change, the function is accusative here.

Потеплее means warmer or a bit warmer.

It is based on the comparative теплее = warmer. The prefix по- often makes the comparative sound softer or less absolute:

• теплее = warmer
• потеплее = a little warmer / somewhat warmer / on the warmer side

In this sentence, шарф потеплее means a warmer scarf.

Russian often uses this kind of comparative in everyday advice:

• оденься потеплее = dress more warmly
• возьми куртку полегче = take a lighter jacket

Because comparative forms like теплее and потеплее are generally indeclinable. They do not change for gender, number, or case.

So you can say:

• шарф потеплее
• куртку потеплее
• носки потеплее

The noun changes as needed, but потеплее stays the same.

More or less, yes, but natural English usually restructures it.

Russian can say:

• Надень шарф потеплее

This means something like:

• Put on a warmer scarf
• Wear a warmer scarf
• Put on a scarf that’s warmer

So the Russian structure is natural, even though the most idiomatic English translation may sound different.

The colon introduces the reason or explanation for the first clause.

So:

• Надень шарф потеплее: = Put on a warmer scarf:
• вечером у реки будет ветрено. = it will be windy by the river in the evening.

In other words, the second part explains why the speaker is giving the advice.

In English, you might also use a colon, dash, or just split it into two sentences.

Вечером is the instrumental singular of вечер, but here it functions as an adverbial expression of time, meaning in the evening.

This is a very common Russian pattern:

• утром = in the morning
• днём = during the day
• вечером = in the evening
• ночью = at night

So although it comes from a noun form, learners should usually treat вечером as a set time expression meaning in the evening.

Because у means by, near, at, and it requires the genitive case.

The noun is река = river. Its genitive singular is реки.

So:

• у реки = by the river / near the river

This is a very common construction:

• у дома = by the house
• у окна = by the window
• у моря = by the sea

Because Russian uses ветрено here as a predicative adverb / category of state word, not as a regular adjective.

So:

• ветреный день = a windy day
  Here ветреный describes a noun.
• сегодня ветрено = it is windy today
  Here ветрено describes the general weather situation.

In the sentence:

• вечером у реки будет ветрено

the meaning is it will be windy by the river in the evening.

Russian does not need a word like English it in this kind of weather expression.

No real subject is needed. This is an impersonal construction.

English says:

• It will be windy

But Russian simply says:

• Будет ветрено

The verb is still in the neuter singular future form будет, but there is no actual subject pronoun.

This is very common with weather and general conditions:

• Было холодно. = It was cold.
• Будет темно. = It will be dark.
• Здесь шумно. = It is noisy here.

Yes. Russian word order is flexible, though the version given sounds natural.

For example, these are also possible:

• Вечером у реки будет ветрено.
• У реки вечером будет ветрено.

The differences are mainly about emphasis and flow, not basic meaning.

In the original sentence, the order feels natural and neutral: first the advice, then the explanation.

The standard stress is:

• Наде́нь шарф потепле́е: ве́чером у реки́ бу́дет ве́трено.

This can be useful because several words may be tricky for learners:

• наде́нь
• потепле́е
• ве́чером
• реки́
• ве́трено

Stress matters a lot in Russian pronunciation, so it is worth learning whole phrases with stress when possible.