У моей племянницы уже есть любимая заколка, но сегодня она просит заплести ей косу.

Russian usually expresses possession with the pattern:

у + possessor in the genitive + есть + thing possessed

So У моей племянницы есть любимая заколка literally means something like At my niece there is a favorite hair clip.

This is the normal Russian way to say my niece has a favorite hair clip.

A few useful notes:

• у here means by / at in a possession construction
• есть literally means there is / exists
• in the present tense, есть is sometimes omitted in casual speech, but here keeping it is perfectly natural

Because they depend on у, which requires the genitive case.

Base forms:

• моя племянница = my niece

After у:

• у моей племянницы = of / at my niece

So:

• племянница → племянницы
• моя → моей

Both words change because the whole phrase is in the genitive.

Because любимая заколка is the thing that exists / is possessed, so it stays in the nominative case.

In the structure у кого-то есть что-то, the possessed thing is usually nominative:

• У меня есть книга = I have a book
• У неё есть сестра = She has a sister
• У моей племянницы есть любимая заколка = My niece has a favorite hair clip

So:

• любимая = nominative feminine singular
• заколка = nominative feminine singular

уже means already.

So:

• У моей племянницы уже есть любимая заколка = My niece already has a favorite hair clip

Its position is fairly natural here: it comes before есть and modifies the whole idea of possession.

Russian word order is flexible, but this placement is very common and neutral.

Yes, Russian often omits pronouns when the subject is obvious, but it can still use them for clarity, contrast, or just normal flow.

Here:

• но сегодня она просит... = but today she asks...

Using она is completely natural.

A learner may also notice that both племянница and заколка are feminine, so in theory она could point to either one. In practice, the meaning makes it clear:

• a niece can ask someone to braid her hair
• a hair clip cannot

So context removes the ambiguity.

This is the pattern:

просить + infinitive = to ask someone to do something

So:

• она просит заплести ей косу = she asks [someone] to braid her hair into a braid / to braid a plait for her

Russian often leaves the person being asked unstated if it is obvious from context.

The infinitive заплести is what she wants done.

Because заплести is perfective, and it refers to a completed result: making the braid.

• плести = imperfective, process-oriented, to be braiding / to braid
• заплести = perfective, result-oriented, to braid up / to finish braiding

Here the idea is not just the process of braiding, but the completed action: she wants someone to braid her hair.

That is why заплести is very natural.

ей is the dative form of она.

Here it marks the person for whom the action is done:

• заплести ей косу = literally to braid a braid for her

This is a very common Russian pattern. Instead of using a possessive like её, Russian often uses the dative pronoun when talking about parts of the body, clothing, hair, and things closely connected to the person.

So Russian prefers something like:

• ей заплести косу

rather than explicitly saying её косу, unless there is a special need to stress ownership.

Because it is the direct object of заплести.

Dictionary form:

• коса = braid / plait

Accusative singular:

• косу

So:

• заплести косу = to braid a braid / to make a braid

Since коса is a feminine noun ending in -а, the accusative singular changes to -у.

Because Russian often omits possessive words when ownership is obvious from context.

In this sentence, ей already tells us whose hair is meant, so косу does not need её.

Compare the natural Russian style:

• Она просит заплести ей косу = natural
• Она просит заплести её косу = possible in some contexts, but it sounds more specific or contrastive, as if you really need to emphasize her braid rather than someone else's

So the version in the sentence is the more natural everyday wording.

Заколка is a general word for a hair clip, barrette, or similar hair accessory.

It does not always refer to one single specific type of clip in the way English sometimes distinguishes between hairpin, clip, barrette, and so on. The exact English translation depends on context.

So любимая заколка is naturally something like:

• favorite hair clip
• favorite barrette

No. Коса has more than one meaning in Russian.

Common meanings include:

• braid / plait
• scythe
• sandspit in geography

In this sentence, because of заплести and the hair context, it clearly means braid / plait.

This is very normal in Russian: one word can have several meanings, and the surrounding words make the intended meaning clear.

сегодня means today, and here it contrasts with the earlier idea:

• she already has a favorite hair clip,
• but today she is asking for a braid.

Placing сегодня before она просит makes that contrast feel natural:

• но сегодня она просит... = but today she asks...

Russian word order is flexible, but this order is neutral and clear. It highlights today as the important time reference.