Если на улице жарко, я надеваю кепку и сандалии.

Если means if.

In this sentence, it introduces a condition: If it’s hot outside, ...

It does not always have to come first. You can also say:

Я надеваю кепку и сандалии, если на улице жарко.
= I put on / wear a cap and sandals if it’s hot outside.

Both word orders are natural. When the если clause comes first, it is followed by a comma.

Russian uses a comma to separate the conditional clause from the main clause.

So in:

Если на улице жарко, я надеваю кепку и сандалии.

the part Если на улице жарко is the if-clause, and я надеваю кепку и сандалии is the main clause.

This is very similar to English:

If it’s hot outside, I wear a cap and sandals.

На улице literally means on the street, but in everyday Russian it often means outside / outdoors.

So на улице жарко is a very common way to say:

• It’s hot outside
• It’s hot outdoors

This is idiomatic Russian. A learner might expect something more directly equivalent to English outside, but на улице is one of the most natural ways to express that idea.

Улице is in the prepositional case.

The basic form is улица. After the preposition на, when talking about location, Russian usually uses the prepositional case:

• улица → nominative
• на улице → prepositional

So:

• на улице = outside / on the street

This is a standard pattern:

• на столе = on the table
• на работе = at work
• на улице = outside / on the street

Жарко here is not an adjective describing a noun. It is a predicative word used to describe a state or condition: it is hot.

Russian often uses words like this for weather, temperature, and general conditions:

• холодно = it is cold
• тепло = it is warm
• жарко = it is hot
• темно = it is dark

So:

На улице жарко = It’s hot outside

If you used жаркая or жаркий, you would need a noun:

• жаркая погода = hot weather
• жаркий день = a hot day

This is a very common learner question.

In standard Russian:

• надевать / надеть = to put on something
• одевать / одеть = to dress someone

So:

• Я надеваю кепку. = I put on a cap.
• Я одеваю ребёнка. = I dress the child.

A useful rule learners are often taught is:

• надеть что? = put on what?
• одеть кого? = dress whom?

So in this sentence, надеваю is correct because the speaker is putting on items of clothing.

Russian present tense can describe a habit, a general rule, or something someone does regularly in that situation.

So:

Если на улице жарко, я надеваю кепку и сандалии.

means something like:

• If it’s hot outside, I put on a cap and sandals
• If it’s hot outside, I wear a cap and sandals

In English, both can work depending on context. In Russian, the present tense here suggests a repeated or habitual action, not just one single moment.

Because кепку is in the accusative case.

The verb надеваю takes a direct object: you put on something. That object goes in the accusative.

The dictionary form is:

• кепка = cap

For a feminine noun ending in -а, the accusative singular usually changes to -у:

• кепка → кепку

So:

• Я надеваю кепку. = I put on a cap.

Сандалии is also in the accusative case, but for inanimate plural nouns, the accusative is often the same as the nominative.

So:

• nominative plural: сандалии
• accusative plural: сандалии

That is why it looks unchanged.

Compare:

• Я вижу столы. = I see tables.
  Here столы is accusative plural, and it looks like nominative plural because tables are inanimate.

So in your sentence:

• кепку = accusative singular
• сандалии = accusative plural

Because that is how these words normally work.

• кепка is a singular countable item: one cap
• сандалии is usually used in the plural, like sandals in English

This is similar to English, where we normally say:

• a cap
• sandals

You usually wear one cap but two sandals, so Russian naturally uses singular for кепка and plural for сандалии.

Yes. Russian word order is flexible, although some versions sound more neutral than others.

The original sentence:

Если на улице жарко, я надеваю кепку и сандалии.

is very natural and neutral.

You could also say:

Я надеваю кепку и сандалии, если на улице жарко.

This puts the main action first and the condition second.

You can also move words around for emphasis, but not every order is equally natural for a beginner. The original version is a good standard model.

Most naturally, it expresses a habit or general practice.

It sounds like:

Whenever it’s hot outside, I wear / put on a cap and sandals.

That is because:

• если sets up a general condition
• надеваю is imperfective present tense, which often describes repeated or usual actions

If you wanted a one-time future meaning, Russian would usually phrase it differently, for example with future forms depending on the context.

They are different in both aspect and often meaning in context.

• надеваю = imperfective, present tense
  Often used for habitual/repeated action: I put on / I wear
• надену = perfective, future
  Usually means a single completed future action: I will put on

So:

• Если на улице жарко, я надеваю кепку и сандалии.
  = general habit

• Если на улице будет жарко, я надену кепку и сандалии.
  = if it is hot outside, I will put on a cap and sandals

That is a very important Russian aspect contrast.

Yes. Russian often does not use a subject like English it in weather and condition expressions.

English says:

• It’s hot
• It’s cold
• It’s dark

Russian often simply says:

• Жарко
• Холодно
• Темно

And if you want to specify where, you add a location:

• На улице жарко = It’s hot outside
• Дома тепло = It’s warm at home

So the lack of a word for it is completely normal in Russian.