Babcia kupiła mi jeszcze jedenaście nowych kredek, bo stare zostały w szkole.

Because Polish past-tense verbs agree with the gender and number of the subject.

The subject here is babcia (grandma), which is feminine singular, so the verb is kupiła.

Compare:

• Babcia kupiła = Grandma bought
• Dziadek kupił = Grandpa bought

So the -ła ending tells you the subject is feminine singular.

Mi is the unstressed dative form of ja (I / me). In this sentence it means to me or for me:

Babcia kupiła mi... = Grandma bought me / bought for me...

Polish often uses the dative like this for the person who benefits from the action.

Mnie is the fuller, stressed form. You would use mnie for emphasis, contrast, or after prepositions. In a neutral sentence like this, mi is more natural.

Here jeszcze means another, an additional, or some more.

So jeszcze jedenaście nowych kredek means another eleven new crayons or eleven more new crayons.

In other contexts, jeszcze can also mean:

• still
• yet
• even

But in this sentence, the idea is clearly additional quantity.

Because in Polish, after numerals 5 and above, the noun usually appears in the genitive plural, and any adjective modifying it also matches that pattern.

So:

• jedna nowa kredka = one new crayon
• dwie nowe kredki = two new crayons
• jedenaście nowych kredek = eleven new crayons

Here:

• nowych = genitive plural of nowe / nowy
• kredek = genitive plural of kredka

This is one of the big differences between Polish and English number phrases.

Kredek is the genitive plural form of kredka (crayon).

Basic forms:

• kredka = nominative singular
• kredki = nominative plural
• kredek = genitive plural

Because jedenaście belongs to the 5+ numeral group, Polish uses the genitive plural noun form after it. That is why you get jedenaście kredek, not jedenaście kredki.

Because the adjective has to match the noun phrase pattern required by jedenaście.

Since kredek is genitive plural, the adjective nowy also becomes genitive plural:

• nowych

So:

• jedenaście nowych kredek

This is adjective agreement: the adjective follows the grammatical form of the noun phrase, not just the meaning.

Because Polish, like English, can sometimes use an adjective by itself when the noun is understood.

Here stare means the old ones, referring to the old crayons. The full version would be:

bo stare kredki zostały w szkole

But since kredki is already obvious from context, Polish leaves it out.

So:

• stare = the old ones

Because the omitted noun is kredki, which is a non-masculine-personal plural noun.

In Polish plural agreement:

• starzy is mainly for masculine-personal groups, like men
• stare is used for non-masculine-personal plurals, including feminine plural nouns like kredki

So if the noun were expressed, it would be:

• stare kredki

That is why the adjective alone is also stare.

Here zostały w szkole means were left at school or ended up staying at school.

The verb zostać often means:

• to remain
• to be left
• to end up

So this clause does not just say that the crayons were in school; it suggests that they stayed there / were left behind there.

That is why zostały is a good choice here.

Because the subject is not a masculine-personal plural group.

The understood subject is stare [kredki]. Since kredki is feminine plural, Polish uses the non-masculine-personal plural past form:

• zostały

Compare:

• chłopcy zostali = the boys stayed / were left
• kredki zostały = the crayons stayed / were left

So -li is for masculine-personal plural, while -ły is for non-masculine-personal plural.

Because the preposition w means in, and when it refers to location, it normally takes the locative case.

So:

• szkoła = school
• w szkole = in school / at school

This is a very common pattern:

• w domu = at home
• w pracy = at work
• w sklepie = in the shop

Yes, Polish word order is fairly flexible, but this version sounds natural and neutral.

A few things are going on:

• Babcia comes first as the topic/subject
• kupiła comes early, which is normal
• mi is a short unstressed pronoun, and these often appear near the beginning of the clause
• jeszcze jedenaście nowych kredek stays together as one quantity phrase

You could change the order for emphasis, but it would slightly shift the focus. For example:

• Babcia jeszcze kupiła mi jedenaście nowych kredek
• Jeszcze jedenaście nowych kredek kupiła mi babcia

These are possible, but the original is the most straightforward.

Yes, bo means because.

It is extremely common in everyday Polish and sounds natural here:

• ..., bo stare zostały w szkole. = ..., because the old ones were left at school.

More formal alternatives include:

• ponieważ
• gdyż

But bo is probably the most normal choice in ordinary speech.

Because Polish does not have articles like English a/an and the.

Polish relies on context instead. So:

• Babcia can mean Grandma or my grandma, depending on context
• stare means the old ones here because the crayons are already known

This is very normal in Polish, even though it feels unusual to English speakers at first.