Я постирала подстилку, потому что после прогулки она стала грязной.

Постирала is the past tense of постирать, which means to wash / to do the washing of something, usually with the idea of completing the action.

The ending -ла tells you the speaker is female. In the Russian past tense, verbs agree in gender and number:

• я постирал = I washed ... (male speaker)
• я постирала = I washed ... (female speaker)
• мы постирали = we washed ...

So this sentence was said by a woman.

Because подстилку is the accusative singular form, used for the direct object of the verb.

The dictionary form is подстилка. Since it is a feminine noun ending in -а, its accusative singular usually changes like this:

• nominative: подстилка
• accusative: подстилку

Here, the thing being washed is the direct object, so Russian uses the accusative:

• Я постирала подстилку = I washed the mat/pad/bedding.

Подстилка is a fairly general word for something laid underneath: a mat, pad, liner, bedding, or animal bed, depending on context.

In this sentence, especially with after the walk, it likely means something like:

• a pet mat
• a dog bed
• a pad
• some kind of bedding/liner

So the exact English translation depends on the situation.

Because потому что means because and introduces a subordinate clause.

Russian normally puts a comma before subordinate clauses:

• Я постирала подстилку, потому что после прогулки она стала грязной.

This is very standard punctuation.

Yes. Потому что is a very common way to say because.

So:

• Я постирала подстилку, потому что...
  = I washed the mat/bedding because...

It is one fixed conjunction. Learners sometimes try to understand потому and что separately, but in this sentence it is best treated as a single expression meaning because.

Because the preposition после (after) requires the genitive case.

The noun прогулка becomes прогулки in the genitive singular:

• nominative: прогулка = walk
• genitive: прогулки = of a walk / after a walk

So:

• после прогулки = after the walk / after a walk

Russian has no articles, so there is no separate word for a or the.

That means после прогулки can mean:

• after a walk
• after the walk

Context tells you which one sounds better in English.

Она refers back to подстилка.

In Russian, nouns have grammatical gender, and pronouns match that gender:

• подстилка is feminine
• so the pronoun is она = she/it

In English, you would say it became dirty, but in Russian the pronoun is grammatically feminine because the noun is feminine.

Because after the verb стать (to become), Russian usually puts the predicate noun or adjective in the instrumental case.

So:

• грязная = dirty (nominative feminine)
• грязной = dirty (instrumental feminine)

That is why you get:

• она стала грязной = it became dirty

This is a very common pattern:

• он стал врачом = he became a doctor
• она стала счастливой = she became happy

Yes, грязной is the adjective грязный (dirty) in the feminine singular instrumental form.

Here is the chain:

• base form: грязный = dirty
• feminine nominative: грязная
• feminine instrumental: грязной

It is feminine because it describes она / подстилка, and instrumental because it follows стала.

Because the subject of that clause is она, which refers to подстилка, a feminine noun.

In the Russian past tense, verbs agree with the subject in gender and number:

• он стал = he became
• она стала = she/it became
• они стали = they became

So:

• она стала грязной = it became dirty

Yes. Стала comes from стать, which is perfective and means became.

That shows a change of state:

• она стала грязной = it became dirty / got dirty

If you said была грязной, that would mean:

• it was dirty

So the difference is:

• стала грязной = it became dirty
• была грязной = it was dirty

In this sentence, стала is used because the mat became dirty after the walk.

Yes, Russian word order is fairly flexible.

The given sentence is completely natural:

• Я постирала подстилку, потому что после прогулки она стала грязной.

But you could also say:

• После прогулки она стала грязной, поэтому я постирала подстилку. = After the walk it became dirty, so I washed it.

You can also move после прогулки around for emphasis:

• Она стала грязной после прогулки.
• После прогулки она стала грязной.

The original order is neutral and natural.

It most naturally means after the walk / after a walk.

Russian often uses a noun where English might use either a noun phrase or a gerund-style expression. So in natural English, depending on context, you might translate it as:

• after the walk
• after going for a walk
• after we got back from the walk

But grammatically, the Russian expression is literally after the walk.

Because once the noun has already been mentioned, Russian often uses a pronoun, just like English does.

So instead of saying:

• ...потому что после прогулки подстилка стала грязной

it says:

• ...потому что после прогулки она стала грязной

Both are possible, but the pronoun sounds more natural when the reference is already clear.

Possibly, yes, depending on style and nuance.

Both постирать and выстирать can mean to wash clothes or fabric items. Very roughly:

• постирать = the common neutral verb for washing laundry or washable fabric items
• выстирать = also wash, often with a stronger sense of washing it thoroughly/out completely

In many everyday contexts, постирала is the most neutral and common choice.

Usually, yes. Постирать is generally used for washing fabric items, laundry, or things that are treated like laundry.

So if подстилка is a washable cloth pad, bedding, or mat, постирала makes sense.

If it were some hard surface item, Russian would probably use another verb such as:

• помыла = washed/cleaned
• почистила = cleaned

So this wording suggests the item is something fabric-like or washable in that way.

Grammatically, yes, but in translation you should usually think it, not she.

Russian third-person pronouns match grammatical gender:

• masculine noun -> он
• feminine noun -> она
• neuter noun -> оно

So for English speakers, it is best to think:

• она here = it, because the noun подстилка is feminine

It does not mean the speaker thinks of the mat as a female being. It is just grammar.