Когда на улице жарко, я беру кепку, а телефон кладу в маленькую сумочку.

Here на улице usually means outside / outdoors, not necessarily on a street in the literal English sense.

So Когда на улице жарко means When it’s hot outside.

Russian often uses на улице in everyday speech for outdoor conditions:

• На улице холодно. = It’s cold outside.
• На улице дождь. = It’s raining outside / There’s rain outside.

Russian often omits the present-tense equivalent of to be.

Also, жарко here is not an adjective agreeing with a noun; it is a predicative word meaning hot in an impersonal expression.

So:

• На улице жарко. = It is hot outside.

There is no need for a word like есть here. In the present tense, Russian normally does not say is in sentences like this.

Because кепка is the direct object of беру (I take), so it goes into the accusative case.

Кепка is a feminine noun ending in -а, and for most feminine nouns like that:

• nominative: кепка
• accusative: кепку

So:

• Я беру кепку. = I take a cap.

This is a very common pattern:

• книга → книгу
• сумка → сумку
• ручка → ручку

Because телефон is a masculine inanimate noun. In Russian, masculine inanimate nouns usually have the same form in the nominative and accusative singular.

So:

• nominative: телефон
• accusative: телефон

That is why:

• Я кладу телефон... = I put the phone...

Compare:

• Я вижу стол. = I see the table.
• Я вижу телефон. = I see the phone.

But with feminine nouns, the accusative often changes:

• сумка → сумку
• кепка → кепку

Because this sentence describes movement into the bag, not location inside the bag.

In Russian:

• в + accusative = motion into something
• в + prepositional = location inside something

So:

• кладу в маленькую сумочку = I put into a small bag
• в маленькой сумочке = in the small bag (location)

Examples:

• Я иду в дом. = I’m going into the house.
• Я в доме. = I’m in the house.

Since the phone is being placed into the bag, Russian uses в маленькую сумочку.

Because both the adjective and the noun must match in gender, number, and case.

The noun сумочка is feminine singular and, here, accusative singular. So the adjective маленький must also take its feminine accusative singular form.

That gives:

• маленькая сумочка = nominative
• маленькую сумочку = accusative

So both words change together:

• маленькая → маленькую
• сумочка → сумочку

Сумочка is a diminutive form of сумка.

Very often, a diminutive can suggest one or more of these:

• small size
• affection
• everyday naturalness
• a slightly cute or lighter tone

So:

• сумка = bag
• сумочка = small bag / little bag / handbag

In this sentence, сумочку suggests a small bag or small purse/handbag.

Кладу is from класть, which is imperfective. It is used here because the sentence describes a habitual/repeated action:

• When it’s hot outside, I take a cap, and I put my phone in a small bag.

This is something the speaker generally does.

If you used положу (from положить, perfective), it would usually sound more like a single completed future action:

• Я положу телефон в сумочку. = I will put the phone in the bag.

So in this sentence, кладу is the natural choice because it describes a routine.

А often links two parts of a sentence while showing a contrast, comparison, or a shift of focus. It does not always mean a strong but.

Here it has a mild contrastive feeling:

• I take a cap, and as for the phone, I put it in a small bag.

It helps separate the two actions:

• беру кепку
• телефон кладу в маленькую сумочку

If you used и, it would sound more like simple addition:

• ...я беру кепку и кладу телефон...

That is possible in some contexts, but а nicely highlights the second item/action as a separate point.

Russian word order is more flexible than English word order. Both versions are grammatical, but they can sound slightly different in emphasis.

• Я кладу телефон в маленькую сумочку = more neutral
• Я телефон кладу в маленькую сумочку = gives extra focus to телефон

In your sentence, the object-first order works well because the sentence contrasts two things:

• кепку — what I take
• телефон — what I put in the bag

So the structure helps organize the information clearly and naturally.

It is most naturally understood as a habitual action.

Russian present tense can describe:

• what is happening now
• what someone usually does
• repeated actions

Here, because of Когда на улице жарко and the general situation, the meaning is:

• Whenever it’s hot outside, I take a cap and put my phone in a small bag.

So although the verbs are in the present tense, the overall sense is habitual.

Sometimes, but the meaning shifts a little.

• Когда на улице жарко... = When/whenever it’s hot outside...
• Если на улице жарко... = If it’s hot outside...

In this sentence, когда sounds very natural because it describes what the speaker usually does in that situation. It suggests a recurring condition.

Если would sound a bit more conditional and less like a regular routine. Not wrong in every context, but когда is a very good choice here.

Yes, learners often notice that. The verb класть has forms that are not very transparent at first:

• infinitive: класть
• я кладу
• ты кладёшь
• он/она кладёт
• мы кладём
• вы кладёте
• они кладут

So the stem changes to клад- in the present tense forms. This is normal for this verb, and it is worth memorizing as a pattern.

A natural stress pattern is:

Когда́ на у́лице жа́рко, я беру́ ке́пку, а телефо́н кладу́ в ма́ленькую су́мочку.

Main word stresses:

• когда́
• у́лица → на у́лице
• жа́рко
• беру́
• ке́пка → ке́пку
• телефо́н
• кладу́
• ма́ленькую
• су́мочка → су́мочку

A learner may also want to notice that Russian unstressed vowels are often reduced in pronunciation, especially о.