Эта кисть слишком старая, поэтому мама купила мне новую.

Эта agrees with кисть, which is a feminine singular noun.

In Russian, words like this change depending on the noun’s:

• gender
• number
• case

The forms of this are:

• этот — masculine
• эта — feminine
• это — neuter
• эти — plural

Since кисть is feminine, Russian uses эта кисть.

A noun ending in ь can be either masculine or feminine, so you often have to learn its gender individually.

Кисть is a feminine noun. You can tell from the agreement in the sentence:

• эта кисть
• старая
• новую

All of those feminine forms show that кисть is feminine.

This is a common issue for learners because the soft sign ь does not by itself tell you the gender with complete certainty.

Кисть can mean different things depending on context, including:

• brush / paintbrush
• hand or wrist in anatomical contexts
• cluster or bunch in some other contexts

In this sentence, it most naturally means brush, probably a paintbrush or similar tool.

So эта кисть means this brush.

Старая is the feminine singular form of old.

It agrees with кисть, which is feminine singular.

Compare:

• старый стол — an old table
• старая кисть — an old brush
• старое окно — an old window
• старые книги — old books

Russian adjectives must match the noun they describe.

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

• слишком старая = too old

Очень means very.

• очень старая = very old

So the sentence says the brush is not just old, but too old, which helps explain why a new one was bought.

The comma separates two clauses:

• Эта кисть слишком старая
• поэтому мама купила мне новую

Here поэтому means therefore / so / that’s why and connects the second clause to the first.

In writing, Russian commonly puts a comma here because the sentence is made of two linked parts with different predicates:

• кисть ... старая
• мама купила

So the comma helps show the pause and logical connection.

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

It introduces a result:

• The brush is too old.
• Therefore, Mom bought me a new one.

It is a very common way to express cause and result in Russian.

Купила is the past tense of купить, which is usually perfective.

Perfective verbs focus on a completed action. Here, the idea is that Mom bought a new one as a completed event.

Why not покупала?

• купила = she bought it, completed action
• покупала = she was buying / used to buy / bought in an ongoing or less result-focused way

In this sentence, the completed result matters, so купила is the natural choice.

Also, Russian past tense agrees with gender:

• купил — he bought
• купила — she bought
• купило — it bought
• купили — they bought

Since мама is feminine, the verb is купила.

Мне is the dative case form of я.

The basic forms are:

• я — I
• меня — me
• мне — to me / for me

In мама купила мне новую, мне means for me or to me.

Russian often uses the dative for the person who receives something or benefits from an action.

So:

• мама купила мне новую = Mom bought me a new one

Новую is the feminine singular accusative form of новый.

It stands for an omitted noun: новую кисть.

Russian often leaves out a noun when it is obvious from context, just as English says a new one instead of repeating a new brush.

So the full idea is:

• мама купила мне новую кисть

But Russian can shorten this to:

• мама купила мне новую

Why accusative? Because it is the direct object of купила.

And because кисть is feminine, the adjective must also be feminine.

Because the noun is already clear from context.

The first clause mentions кисть, so in the second clause Russian can simply say новую, meaning a new one.

This is very natural in Russian. English does something similar:

• This brush is too old, so Mom bought me a new one.

Russian just uses the adjective by itself, where English usually adds one.

So:

• новую = a new one
• understood noun = кисть

Not explicitly. In the first clause, кисть is in the nominative because it is the subject:

• Эта кисть слишком старая
  = This brush is too old

In the second clause, the noun is not repeated, but the adjective новую shows what case the missing noun would have:

• implied form: новую кисть

That would be accusative singular.

So the sentence contains:

• кисть in nominative, stated directly
• кисть in accusative, only implied through новую

In the present tense, Russian usually omits the verb to be.

So:

• Эта кисть слишком старая literally looks like This brush too old
• but it means This brush is too old

This is completely normal in Russian.

Compare:

• Он врач — He is a doctor
• Книга интересная — The book is interesting
• Эта кисть слишком старая — This brush is too old

If you want past or future, Russian does use forms of to be:

• Эта кисть была слишком старой — This brush was too old

Not in this sentence.

After купила, the thing bought is the direct object, so it must be in the accusative case.

Because the implied noun is feminine singular (кисть), the adjective must also be feminine singular accusative:

• новую

Новая is nominative, not accusative, so it would not fit here.

Compare:

• Новая кисть дорогая. — The new brush is expensive.
  Here новая is nominative because it describes the subject.

• Мама купила новую кисть. — Mom bought a new brush.
  Here новую is accusative because it is the object.

Yes, it is natural.

A fully expanded version would be:

• Эта кисть слишком старая, поэтому мама купила мне новую кисть.

But repeating кисть would sound unnecessary, so native speakers normally shorten it to:

• Эта кисть слишком старая, поэтому мама купила мне новую.

That sounds smooth and natural.