Моя сестра взяла с собой шорты, потому что на даче будет жарко.

Because моя has to agree with сестра in gender, number, and case.

• сестра is feminine
• singular
• nominative case here, because she is the subject of the sentence

So the possessive pronoun must also be feminine singular nominative:

• мой брат = my brother
• моя сестра = my sister
• моё платье = my dress
• мои друзья = my friends

Russian adjectives and possessive words change form much more than in English.

In the past tense, Russian verbs agree with the subject in gender and number.

The subject here is сестра, which is feminine singular, so the verb is:

• взял = he took
• взяла = she took
• взяло = it took
• взяли = they took

So:

• Моя сестра взяла... = My sister took...

This is one of the big differences from English: in Russian, past-tense verbs show gender.

This is about aspect.

• взять / взяла = perfective → a completed action
• брать / брала = imperfective → the process, repeated action, or general fact

Here the sentence describes one completed action: she took the shorts with her. So взяла is the natural choice.

Compare:

• Она взяла шорты. = She took the shorts.
  
  • one completed action
• Она брала шорты каждый раз. = She used to take shorts every time.
  
  • repeated/habitual action

С собой means with oneself / along with oneself.

So:

• взять шорты = to take the shorts
• взять шорты с собой = to take the shorts along / to bring the shorts with you

It adds the idea that she is taking them to another place, not just picking them up.

The word собой comes from the pronoun meaning oneself, and in this expression it changes depending on who the subject is only in meaning, not in form:

• Я взял с собой... = I took ... with me
• Она взяла с собой... = She took ... with her
• Мы взяли с собой... = We took ... with us

This expression is extremely common in Russian.

Yes. Шорты is normally used as a plural-only noun, very much like English shorts.

So you usually say:

• эти шорты = these shorts
• новые шорты = new shorts
• взяла шорты = took shorts / took the shorts

Even though it refers to one item of clothing, grammatically it behaves as plural.

This is similar to other Russian clothing words such as:

• брюки = trousers/pants
• джинсы = jeans
• ножницы = scissors

In Russian, на даче is the normal idiomatic expression for at the dacha / at the country house.

A дача is not just a building; it often means the whole place or property where people spend time in the summer. Because of that, Russian commonly uses на rather than в.

So:

• на даче = at the dacha
• ехать на дачу = to go to the dacha
• быть на даче = to be at the dacha

Using в даче would usually sound wrong in this context.

Also note the case:

• на даче = prepositional case after на when talking about location

Because the heat is in the future.

The sentence means that it will be hot at the dacha, so Russian uses the future form of быть:

• будет = will be

So:

• На даче жарко. = It is hot at the dacha.
• На даче будет жарко. = It will be hot at the dacha.

This matches the logic of the sentence: she took shorts because later, when she is at the dacha, the weather will be hot.

Жарко here is a predicative word used for weather, temperature, and general states. It means hot in the sense of the environment feels hot.

So:

• На улице жарко. = It’s hot outside.
• В комнате жарко. = It’s hot in the room.

Why not жаркая?

• жаркая is an adjective form that would need a noun:
  • жаркая погода = hot weather
  • жаркая комната would be unusual in meaning

Why not горячо?

• горячо usually means hot in the sense of physically hot to the touch, or sometimes emotionally/intensely
• for weather and room temperature, жарко is the normal word

So in this sentence, будет жарко is exactly what Russian normally says for it will be hot.

Because потому что introduces a subordinate clause meaning because.

Russian normally puts a comma before it:

• Моя сестра взяла с собой шорты, потому что на даче будет жарко.

The first part is the main clause:

• Моя сестра взяла с собой шорты

The second part explains the reason:

• потому что на даче будет жарко

This comma is standard Russian punctuation.

Yes. Russian word order is relatively flexible, although each version can sound slightly different in emphasis.

The original sentence is very natural:

• Моя сестра взяла с собой шорты, потому что на даче будет жарко.

You could also say:

• Моя сестра взяла шорты с собой, потому что на даче будет жарко.
• Потому что на даче будет жарко, моя сестра взяла с собой шорты.

These all make sense, but the original is probably the most neutral and natural for everyday speech.

Russian word order often changes for focus and style, not just grammar.

No, but it is the most common and straightforward choice.

Some alternatives are:

• так как = since / because
• поскольку = since / inasmuch as
• sometimes just a different sentence structure

For example:

• Моя сестра взяла с собой шорты, так как на даче будет жарко.

That is grammatical, but потому что sounds more everyday and conversational here.

So for a learner, потому что is the best default way to express because in this kind of sentence.