Я купила новый чехол для телефона, потому что старый порвался.

Because купила is the past tense, feminine singular form of купить (to buy, perfective).

• купил = he bought / I bought if the speaker is male
• купила = she bought / I bought if the speaker is female
• купило = it bought
• купили = they bought / you bought (plural)

In Russian past tense, the verb agrees with the gender and number of the subject, not with person the way English does. So if a woman says Я купила..., it means I bought...

Yes, it could often be omitted if the context is clear:

• Я купила новый чехол...
• Купила новый чехол...

Both can mean I bought a new case...

However, Russian past tense forms like купил / купила / купили show gender and number, but not clearly person in the same way present-tense forms do. So Я is often included to make it obvious that the subject is I, especially when the sentence starts a new conversation or needs emphasis.

Because купила comes from купить, the perfective verb, which presents the action as completed: the speaker successfully bought the case.

• купить = to buy (completed action)
• покупать = to buy / be buying (process, repeated action, or general habit)

Here the speaker means a single finished purchase, so купила is the natural choice.

Compare:

• Я купила новый чехол. = I bought a new case.
• Я покупала новый чехол. = I was buying / I bought (in some context-focused sense, often emphasizing process or experience rather than result)

Чехол is in the accusative singular, because it is the direct object of купила.

But there is an important detail: чехол is an inanimate masculine noun, so its accusative form is the same as the nominative.

• nominative: новый чехол
• accusative: новый чехол

So although it is functioning as the object, the form does not change.

Because the adjective новый (new) must agree with the noun чехол in:

• gender: masculine
• number: singular
• case: accusative (same form as nominative here because the noun is inanimate)

So:

• новый чехол = new case

If the noun were feminine, plural, or in another case, the adjective would change too.

Because the preposition для (for) requires the genitive case.

So:

• nominative: телефон
• genitive: телефона

That is why Russian says:

• чехол для телефона = a case for a phone / phone case

This is a very common pattern:

• подарок для друга = a gift for a friend
• вода для чая = water for tea

Yes, you may hear both, but there is a slight nuance.

• чехол для телефона = a case for a phone; neutral and standard
• чехол на телефон = a case for / that goes on a phone; very common in everyday speech, often when talking about compatibility with a specific device

In many real-life contexts, both are understood. Для телефона sounds a bit more neutral/dictionary-like; на телефон can sound more colloquial and product-oriented.

In Russian, an adjective can sometimes stand alone and mean the ... one, if the noun is obvious from context.

So:

• новый чехол = new case
• старый = the old one

Here старый really means старый чехол (the old case), but Russian leaves out the repeated noun because it is already clear.

English does something similar with words like:

• the rich
• the poor
• the old one

Because it refers back to чехол, which is a masculine noun.

Even though чехол is omitted in the second clause, the adjective still keeps the gender, number, and case that match the missing noun:

• старый (чехол) = the old case

If the omitted noun were feminine, you would need старая instead.

Порвался comes from порваться, which means to tear, to rip, or to get torn.

So старый порвался means:

• the old one tore
• the old one got ripped
• the old one ripped

It describes something made of soft material, fabric, leather, plastic, etc., becoming torn. For a phone case, this is natural if the case is flexible or has worn out.

Because порваться is a reflexive/intransitive verb, and -ся changes the meaning.

• порвать = to tear something
• порваться = to tear / get torn

Compare:

• Я порвала чехол. = I tore the case.
• Чехол порвался. = The case tore / got torn.

So in this sentence, the old case is not doing the action to something else; it is the thing that became torn. That is why порвался is used.

Because it agrees with the subject, which is старый (чехол).

Russian past tense verbs agree in gender and number:

• порвался = masculine singular
• порвалась = feminine singular
• порвалось = neuter singular
• порвались = plural

Since чехол is masculine singular, the verb must be порвался.

Because потому что introduces a subordinate clause (because ...), and Russian normally separates that clause with a comma.

So:

• Я купила новый чехол для телефона, потому что старый порвался.

This is the standard punctuation pattern.

Yes. Russian word order is flexible, although some orders sound more neutral than others.

The given sentence is a very natural neutral order:

• Я купила новый чехол для телефона, потому что старый порвался.

You could also say:

• Потому что старый порвался, я купила новый чехол для телефона.

That puts more focus on the reason first.

You can also omit Я if the context is clear:

• Купила новый чехол для телефона, потому что старый порвался.

So the meaning stays the same, but the emphasis can shift.

Because Russian has no articles.

English distinguishes:

• a new case
• the old case

Russian does not use separate words for that. Instead, definiteness is understood from context.

So:

• новый чехол can mean a new case or the new case
• старый can mean the old one here because the context makes it specific

It is the most common and straightforward way to say because in everyday Russian.

• потому что = because

There are other ways to express cause, such as так как or из-за того что, but потому что is the most common choice in ordinary speech and writing.

So this sentence sounds very natural as it is.