Эта помада слишком тёмная, поэтому я взяла другую и положила её в косметичку.

Because they describe помада, and помада is a feminine noun in Russian.

In Russian, words that agree with a noun usually match it in:

• gender
• number
• case

Here:

• эта помада = this lipstick
• тёмная = dark

Since помада is feminine singular and is the subject of the clause, the adjective and demonstrative stay in the feminine singular nominative form:

• эта
• тёмная

That ending shows it is a feminine singular nominative adjective.

The basic dictionary form of many adjectives is the masculine:

• тёмный = dark

To match a feminine noun like помада, it changes to:

• тёмная

So:

• тёмный карандаш = a dark pencil
• тёмная помада = a dark lipstick

Слишком means too in the sense of excessively or more than is desirable.

So:

• слишком тёмная = too dark

This is different from simply saying very dark.
For example:

• очень тёмная = very dark
• слишком тёмная = too dark

In the past tense, Russian verbs agree in gender with the speaker or subject.

So if the speaker is female:

• я взяла
• я положила

If the speaker were male:

• я взял
• я положил

Russian past tense does this regularly:

• masculine: usually no final -а
• feminine: usually ends in -а

This sentence suggests the speaker is a woman.

Because it is the direct object of взяла, so it has to be in the accusative case.

The full idea is:

• я взяла другую помаду = I took another lipstick

Since помаду would be feminine singular accusative, другую also becomes feminine singular accusative.

Compare:

• nominative: другая помада
• accusative: другую помаду

In the sentence, the noun помаду is omitted because it is obvious from context, so only другую remains.

Yes. This is very common in Russian.

When the noun is obvious, Russian often leaves it out:

• я взяла другую = I took another one

The omitted noun is understood from context:

• другую [помаду]

English does something similar with another one.

Её is the pronoun her/it, and here it means it, referring to the lipstick.

In this sentence:

• положила её в косметичку = put it in the makeup bag

Because помада is feminine in Russian, the pronoun referring to it is also feminine:

• её

A useful thing to know: её can be both:

• genitive
• accusative

Here it is accusative, because it is the direct object of положила.

In this sentence, keeping её is the most natural way to show what was put into the bag.

Russian can sometimes omit objects when they are obvious, but here её helps the sentence flow clearly:

• я взяла другую и положила её в косметичку

Without её, the sentence may sound abrupt or less clear, especially for learners. Native speakers might omit pronouns in some contexts, but using её here is completely normal and natural.

Because в can take different cases depending on the meaning.

Here the meaning is movement into something:

• положила в косметичку = put into the makeup bag

With movement toward/into a destination, в usually takes the accusative:

• в косметичку

If you were talking about location, meaning in the makeup bag, you would use the prepositional:

• в косметичке = in the makeup bag

Compare:

• она положила помаду в косметичку = she put the lipstick into the bag
• помада лежит в косметичке = the lipstick is in the bag

It is accusative singular.

The noun is:

• косметичка = makeup bag / cosmetic bag

For many feminine nouns ending in -а, the accusative singular changes to -у:

• nominative: косметичка
• accusative: косметичку

That is why after в with motion, you get:

• в косметичку

Because поэтому connects two clauses, and in normal Russian punctuation, a comma is used between them.

Here the sentence has two parts:

• Эта помада слишком тёмная
• поэтому я взяла другую и положила её в косметичку

So the comma marks the boundary between:

1. the reason/situation
2. the result

It is similar to English punctuation in a sentence like:

• This lipstick is too dark, so I took another one...

Поэтому means therefore, so, or that’s why.

It introduces a result:

• Эта помада слишком тёмная, поэтому...
• This lipstick is too dark, so...

It is different from потому что, which means because and introduces a reason, not a result.

Compare:

• Она взяла другую, потому что эта помада слишком тёмная.
  She took another one because this lipstick is too dark.
• Эта помада слишком тёмная, поэтому она взяла другую.
  This lipstick is too dark, so she took another one.

Because the sentence describes completed actions:

• she took another one
• she put it in the bag

Russian often uses the perfective aspect for single, completed events in a sequence.

Here:

• взять = to take (perfective)
• положить = to put/place (perfective)

Their past feminine forms are:

• взяла
• положила

If you used imperfective forms, the meaning would shift toward process, repetition, or background context, which would not fit as well here.

No, Russian word order is fairly flexible, though the original version is very natural.

The given sentence is neutral and clear:

• Эта помада слишком тёмная, поэтому я взяла другую и положила её в косметичку.

You can move parts around for emphasis, for example:

• Поэтому я взяла другую и положила её в косметичку.
• Другую я взяла и положила её в косметичку.

But changing the order can change the emphasis or make the sentence sound less neutral. For learners, the original order is a very good model to follow.