Po południu idę do magazynu, bo brakuje nam kopert i taśmy.

This is a fixed time expression in Polish.

• po południu means in the afternoon
• grammatically, po is followed here by the locative form of południe
• literally, it comes from the idea of after noon

So:

• rano = in the morning
• wieczorem = in the evening
• po południu = in the afternoon

You should learn po południu as one whole expression.

Polish makes an important distinction with verbs of motion.

• idę = I am going / I’m going now / I’m going in one specific direction
• chodzę = I go regularly / I walk around / I go habitually

In this sentence, the speaker is talking about one planned trip to the warehouse, so idę is the natural choice.

Compare:

• Codziennie chodzę do magazynu. = I go to the warehouse every day.
• Dziś idę do magazynu. = Today I’m going to the warehouse.

In Polish, the present tense of an imperfective verb is often used for a planned or near-future action, especially when the time is already clear.

Here, Po południu tells us when it will happen, so idę naturally means something like I’m going this afternoon.

This is very similar to English:

• I’m going this afternoon.

So although idę is grammatically present tense, the time phrase makes it future in meaning.

Because do expresses movement to a place, and do takes the genitive case.

• dictionary form: magazyn
• after do: do magazynu

So:

• idę do magazynu = I’m going to the warehouse / storeroom

Compare:

• jestem w magazynie = I am in the warehouse
• idę do magazynu = I am going to the warehouse

Also, note that magazyn usually means warehouse, storeroom, or stockroom in this kind of context, not the English word magazine meaning a periodical.

In this sentence, magazyn most likely means a storeroom, stockroom, or warehouse—a place where office or work supplies are kept.

It does not usually mean an English magazine in this context.

Polish magazyn can mean different things depending on context, but with office supplies like koperty and taśma, it strongly suggests a storage room.

Because in standard Polish, a comma is normally required before bo.

• bo = because

So:

• Idę do magazynu, bo brakuje nam kopert.

This is a normal punctuation rule in Polish. English learners sometimes want to omit the comma because English punctuation works differently, but in Polish the comma before bo is expected.

Both can mean because, but they are used a little differently.

• bo is very common and natural in everyday speech
• ponieważ is more formal or more written

So in this sentence, bo sounds completely natural.

You could also say:

• Po południu idę do magazynu, ponieważ brakuje nam kopert i taśmy.

That is correct, just a bit more formal in tone.

brakować means to be lacking / to be missing / to be in short supply.

So brakuje nam... literally works like:

• there is a lack of ... to us

That is why:

• nam is in the dative
• the missing thing is in the genitive

This structure is very common in Polish.

Compare the nuance:

• Nie mamy kopert i taśmy. = We don’t have envelopes and tape.
• Brakuje nam kopert i taśmy. = We’re short of envelopes and tape / We’re lacking envelopes and tape.

The second version often sounds more natural when talking about supplies that need replenishing.

nam is the dative form of my = we/us.

With brakować, the person affected by the shortage goes in the dative:

• brakuje mi = I’m lacking / I’m short of
• brakuje ci = you’re lacking
• brakuje nam = we’re lacking
• brakuje im = they’re lacking

So:

• brakuje nam kopert = we are short of envelopes

This is just the case pattern that brakować requires.

Because brakować takes the missing thing in the genitive case, not the nominative.

The noun is:

• koperta = envelope

Its plural forms include:

• koperty = nominative plural
• kopert = genitive plural

Since the sentence uses brakuje, you need the genitive:

• brakuje nam kopert

If you said koperty, that would not fit the grammar of brakować here.

For the same reason: brakować requires the genitive.

The basic noun is:

• taśma = tape

Here, taśmy is the genitive singular form, which makes sense if tape is being treated as an uncountable material or supply.

So:

• brakuje nam taśmy = we’re short of tape

This is very natural in Polish.

A useful detail: taśmy can also be a different case/form in other contexts, but here the grammar and meaning show that it is genitive singular.

Because this construction is usually treated as impersonal.

In brakuje nam kopert i taśmy, the verb does not agree the way an ordinary English verb would. The focus is on the idea that there is a shortage of something.

So Polish normally uses:

• brakuje rather than a plural form here

That is standard and natural.

Yes. Polish word order is fairly flexible, although different orders can sound more or less natural depending on emphasis.

The original:

• Po południu idę do magazynu, bo brakuje nam kopert i taśmy.

Other possible versions:

• Idę po południu do magazynu, bo brakuje nam kopert i taśmy.
• Bo brakuje nam kopert i taśmy, po południu idę do magazynu.

The original version sounds very natural because it begins with the time expression and then gives the reason.

So yes, the order can change, but the original is a good standard model to learn from.