У моей сестры длинная коса.

A very common Russian way to express possession is:

у + possessor in the genitive + possessed thing

So:

• У моей сестры длинная коса literally feels like At my sister, there is a long braid
• Natural English translation: My sister has a long braid

This pattern is much more common in everyday Russian than using a verb like иметь (to have).

So instead of saying Моя сестра имеет длинную косу, Russian usually prefers У моей сестры длинная коса.

Because the preposition у requires the genitive case.

Base forms:

• моя сестра = my sister

After у, they change to genitive:

• моя → моей
• сестра → сестры

So:

• у моей сестры = of / at my sister

This is just the normal case pattern after у when showing possession.

Because длинная describes коса, and коса is:

• feminine
• singular
• nominative

So the adjective has to agree with it:

• длинный — masculine
• длинная — feminine
• длинное — neuter
• длинные — plural

Since коса is feminine singular, the correct form is длинная коса.

In this kind of possession sentence, the thing being possessed is usually in the nominative case.

So in:

• У моей сестры длинная коса

the possessed thing is коса, and it appears in nominative:

• коса = a braid / plait

This is because the sentence is structurally like there is a long braid at my sister’s.

So:

• possessor after у → genitive
• thing possessed → nominative

Yes. Коса is a word with multiple meanings. It can mean:

• braid / plait (hair)
• scythe
• sandspit or narrow strip of land

In this sentence, because of сестра and длинная, the meaning is clearly braid (a long plait of hair).

So У моей сестры длинная коса means your sister has long braided hair / a long braid, not that she owns a scythe.

Because Russian usually does not use a present-tense verb equivalent to has in this structure.

English:

• My sister has a long braid

Russian:

• У моей сестры длинная коса

There is no separate present-tense verb here. Russian simply uses the possession construction without есть in most ordinary statements.

You may learn есть as is/there is, but in everyday present-tense possession sentences it is often omitted:

• У меня брат = I have a brother
• У неё машина = She has a car
• У моей сестры длинная коса = My sister has a long braid

Yes, it is grammatically possible, but it is usually less natural in ordinary speech.

• У моей сестры длинная коса = natural, everyday Russian
• Моя сестра имеет длинную косу = more formal, bookish, or unusual in normal conversation

Russian does have the verb иметь (to have), but it is not the default choice for simple possession the way have is in English.

So for a learner, the safest natural choice is:

• У моей сестры длинная коса

Yes. Russian word order is flexible, though the neutral order here is:

• У моей сестры длинная коса

This sounds natural and unmarked.

You could also hear:

• Длинная коса у моей сестры

But that changes the emphasis a bit. It may sound more contrastive or stylistic, as if focusing on long braid first.

For learners, use:

• У моей сестры длинная коса

as the standard version.

In Russian, adjectives usually come before the noun:

• длинная коса = long braid

Putting the adjective after the noun is possible in some contexts, but it is usually:

• more literary,
• more emphatic,
• or less neutral.

So:

• длинная коса = normal
• коса длинная = possible, but usually sounds like emphasis or contrast, such as The braid is long

In this sentence, длинная коса is the natural choice.

The usual negative version is:

• У моей сестры нет длинной косы.

Notice two important changes:

1. нет is used for there is not / does not have
2. The possessed thing goes into the genitive:

• коса → косы
• длинная → длинной

So:

• У моей сестры длинная коса. = My sister has a long braid.
• У моей сестры нет длинной косы. = My sister does not have a long braid.

This genitive after нет is very important in Russian.

The stresses are:

• у моей́ сестры́ дли́нная коса́

More specifically:

• моей́
• сестры́
• дли́нная
• коса́

This matters because Russian stress is unpredictable and can change from word to word. A learner should try to memorize the words together with stress.

A rough pronunciation guide:

• у ма-ЕЙ сис-тРЫ ДЛИН-на-я ка-СА

Also remember that unstressed vowels are often reduced in actual pronunciation.

Yes, there is a slight nuance.

Literally, коса is a braid / plait — an actual braid of hair.

So the most direct translation is:

• My sister has a long braid

Depending on context, English might also express the idea more naturally as:

• My sister wears her hair in a long braid
• My sister has long braided hair

But the Russian sentence itself specifically refers to a braid, not just hair length in general. If you wanted to say simply that her hair is long, Russian would more likely use волосы:

• У моей сестры длинные волосы = My sister has long hair