Я взяла только тушь и помаду, потому что на улице уже было жарко.

Because the speaker is female.

In the past tense, Russian verbs change for gender in the singular:

• взял = a man took
• взяла = a woman took
• взяло = it took
• взяли = they took

So Я взяла means I took, said by a woman.

Взяла is from взять, which is the perfective partner of брать.

• брать / брала = to take, was taking, used to take, took in a more process-oriented or repeated sense
• взять / взяла = to take in the sense of a completed action

Here, the speaker means she made one completed choice and took those items. So взяла is the natural form.

Both words are direct objects, so they are in the accusative case. But different noun types change differently.

• тушь is a feminine noun ending in -ь. Its accusative singular is the same as the nominative singular: тушь
• помада is a feminine noun ending in -а. In the accusative singular, -а changes to -у: помаду

So:

• тушь → тушь
• помада → помаду

They are in the accusative case because they are the things the speaker took.

The verb взять normally takes a direct object in the accusative:

• взять книгу = to take a book
• взять сумку = to take a bag
• взять тушь и помаду = to take mascara and lipstick

So the sentence uses the accusative because these are the items being taken.

Только means only.

In this sentence, it limits what she took:

• Я взяла только тушь и помаду = I took only mascara and lipstick

The idea is that she did not take other things.

Its position matters too: here it naturally applies to тушь и помаду as the set of things taken.

Потому что means because.

It introduces the reason:

• Я взяла только тушь и помаду = I only took mascara and lipstick
• потому что на улице уже было жарко = because it was already hot outside

Learners often notice that потому by itself can mean something like for that reason, but in this sentence you need the full conjunction потому что to mean because.

Because жарко is not an adjective here. It is a predicative adverb/state word meaning hot in an impersonal expression.

Russian often says weather and general conditions this way:

• было холодно = it was cold
• было темно = it was dark
• было жарко = it was hot

This kind of expression usually uses neuter singular past tense: было.

So было жарко literally works like it was hot, even though Russian does not need a word for it here.

На улице literally means on the street, but very often it simply means outside / outdoors.

So:

• На улице жарко = It’s hot outside
• На улице дождь = It’s raining outside / There’s rain outside

This is a very common Russian way to talk about outdoor weather or conditions.

Уже means already.

It shows that by that time, the weather had already become hot:

• было жарко = it was hot
• уже было жарко = it was already hot

This suggests the heat was noticeable earlier than expected or earlier in the day.

Yes, Russian word order is fairly flexible, but the original order is very natural.

• Я взяла только тушь и помаду, потому что на улице уже было жарко.

This sounds neutral and clear.

Some parts can move for emphasis, for example:

• Только тушь и помаду я взяла... = emphasizes what exactly was taken
• Уже было жарко на улице... = puts more focus on already

But for a learner, the original version is the best standard word order.

Not always. Russian often drops subject pronouns when the verb already makes the subject clear.

So you could also say:

• Взяла только тушь и помаду, потому что на улице уже было жарко.

This still clearly means I took only mascara and lipstick..., especially in context.

However, Я is perfectly correct and can be used for clarity or emphasis.