Моя попутчица попросила у стюардессы воду.

Because попутчица is a feminine noun.

In Russian, possessive words like my must agree with the noun in gender, number, and case:

• мой = masculine singular
• моя = feminine singular
• моё = neuter singular
• мои = plural

So:

• мой попутчик = my male fellow traveler
• моя попутчица = my female fellow traveler

Here, попутчица is feminine, so моя is required.

Попутчица means a female fellow traveler / travel companion / person traveling with you or alongside you.

It comes from the same idea as:

• попутчик = male fellow traveler
• попутчица = female fellow traveler

Depending on context, it could mean:

• someone traveling on the same trip
• someone seated near you
• someone going in the same direction

So in this sentence, моя попутчица is something like my fellow traveler or the woman traveling with me.

Попросила is the past tense, feminine singular form of попросить.

That tells you two things:

1. The action happened in the past
2. The subject was feminine

Compare:

• он попросил = he asked
• она попросила = she asked
• они попросили = they asked

Since попутчица is feminine, the verb must match it:

• Моя попутчица попросила...

This is about aspect, which is very important in Russian.

• просить = imperfective
• попросить = perfective

Here:

• попросила suggests a single completed request
• просила would suggest was asking, used to ask, or focus more on the process rather than the completed act

In this sentence, the idea is that she asked for water once, so попросила is natural.

Very roughly:

• Она просила воду. = She was asking for water / She asked for water (less focused on completion, and less natural here)
• Она попросила воду. = She asked for water (a completed request)

Because Russian often uses the pattern:

• просить / попросить у кого? что?

That means:

• to ask from whom? for what?

So in this sentence:

• у стюардессы = from the flight attendant
• воду = water

The preposition у requires the genitive case, which is why стюардесса changes to стюардессы.

Dictionary form:

• стюардесса = flight attendant

Genitive singular:

• у стюардессы = from the flight attendant

Yes — стюардессы can look like either:

• genitive singular
• or nominative plural

But here it is genitive singular, because it comes after у, and у requires the genitive.

So:

• стюардесса = a flight attendant
• у стюардессы = from the flight attendant

This is a very common pattern in Russian: the same ending can have more than one function, and the context tells you which one it is.

Because вода is the direct object of the verb, and direct objects often go in the accusative case.

• вода = nominative
• воду = accusative

She asked for what?

• воду

So Russian changes the form:

• вода → воду

This is a very common feminine noun pattern:

• книга → книгу
• машина → машину
• вода → воду

Yes:

• Моя = my
• попутчица = female fellow traveler
• попросила = asked
• у = from
• стюардессы = the flight attendant
• воду = water

So the structure is basically:

• [My fellow traveler] [asked] [from the flight attendant] [water].

Natural English would usually say:

• My fellow traveler asked the flight attendant for water.

Even though Russian uses у (from) here, English normally says ask someone for something.

Because that would not be the normal structure for asking someone for something.

Russian distinguishes between two ideas:

1. asking someone for an object

   • попросить у кого что
   • попросить у стюардессы воду
   • ask the flight attendant for water

2. asking someone to do something

   • попросить кого + infinitive
   • попросить стюардессу принести воду
   • ask the flight attendant to bring water

So:

• попросила у стюардессы воду = asked the flight attendant for water
• попросила стюардессу принести воду = asked the flight attendant to bring water

That is an important difference.

No, Russian word order is fairly flexible.

The sentence as given:

• Моя попутчица попросила у стюардессы воду.

is a neutral, natural order.

But Russian can change word order to shift focus or emphasis. For example:

• У стюардессы моя попутчица попросила воду.
• Воду моя попутчица попросила у стюардессы.

These all mean roughly the same basic thing, but the emphasis changes.

For learners, the given order is a good standard pattern:

• subject + verb + у кого + object

Because Russian has no articles.

So Russian does not have direct equivalents of:

• a
• an
• the

Whether you translate something as a flight attendant or the flight attendant depends on the context.

So:

• стюардесса / стюардессы can mean a flight attendant or the flight attendant, depending on the situation.

In this sentence, English would usually say:

• the flight attendant

because on a plane, the listener can easily understand which person is meant.

You would change both the noun and the past-tense verb form:

• Мой попутчик попросил у стюардессы воду.

Changes:

• моя → мой
• попутчица → попутчик
• попросила → попросил

That is because Russian marks gender not only on some nouns and adjectives, but also in the past tense.