I september köper vi nya block och pennor till skolan.

Because Swedish uses i for months to mean in or during a month.

• i september = in September
• i januari = in January
• i sommar = in the summer

So I september is the normal way to begin a sentence like this.

In Swedish, names of months are normally not capitalized.

So Swedish writes:

• september
• oktober
• måndag

This is different from English, where September and Monday are capitalized.

This is because Swedish main clauses follow the V2 rule: the finite verb usually comes in second position.

Here, I september has been placed first. That means the verb must come next:

• I september köper vi nya block och pennor till skolan.

If you start with the subject instead, you get:

• Vi köper nya block och pennor till skolan i september.

Both are correct, but the word order changes because of the V2 rule.

Köper is the present tense of köpa = to buy.

Swedish often uses the present tense for future plans or scheduled actions, especially when there is a time expression like I september.

So this can mean something like:

• we buy in September
• we are buying in September
• we will buy in September

If you want to make the future meaning more explicit, Swedish can also use ska:

• I september ska vi köpa nya block och pennor till skolan.

But the present tense is very natural here.

Nya is the plural form of the adjective ny = new.

It has to match the nouns it describes:

• en ny penna = a new pen
• ett nytt block = a new notebook/pad
• nya block och pennor = new notebooks/pads and pens

Since block and pennor are plural together, Swedish uses nya.

Because block is one of the Swedish nouns whose indefinite singular and indefinite plural look the same.

• ett block = a notebook/pad
• två block = two notebooks/pads

This is normal for many ett-words.

The definite forms are different:

• blocket = the notebook/pad
• blocken = the notebooks/pads

The singular is penna.

• en penna = a pen
• pennor = pens

So pennor is the indefinite plural form.

This is a very common plural pattern for en-words:

• en flicka → flickor
• en penna → pennor

Skolan is the definite form of skola:

• en skola = a school
• skolan = the school

After till, Swedish very often uses the definite form when talking about a specific destination or the school context that is understood.

So till skolan is the natural expression here.

English often says for school without the, but Swedish commonly says till skolan.

It can suggest either idea depending on context, but here it most naturally means for school or for use at school.

So the sentence is about buying school supplies.

Still, Swedish uses till in a way that can feel more like to than English does. That is why till skolan may look more literal than the English translation.

Because Swedish usually does not use an article in the indefinite plural.

Compare:

• en penna = a pen

• pennor = pens

• ett block = a notebook/pad
• block = notebooks/pads

So nya block och pennor simply means new notebooks/pads and pens, with no word corresponding to some or the unless you specifically want that meaning.