Die beker by die venster is ons s'n, maar die bord by die deur is hulle s'n.

die is the normal definite article in Afrikaans, meaning the.

Afrikaans does not change the article for grammatical gender, so the same word is used with beker, venster, bord, and deur. It is also used with plural nouns, so die is one of the most common words in the language.

Here by means by, near, or next to.

So:

• die beker by die venster = the cup by the window
• die bord by die deur = the plate by the door

It introduces a prepositional phrase that tells you where something is.

Because by die venster is directly describing die beker.

It works just like English the cup by the window: the location phrase helps identify which cup is meant. The same is true for die bord by die deur.

In a normal Afrikaans main clause, the finite verb usually comes second.

The whole subject Die beker by die venster counts as one unit, so the structure is:

Die beker by die venster + is + ons s'n

The second clause follows the same pattern:

die bord by die deur + is + hulle s'n

They mean ours and theirs.

Afrikaans often uses s'n to make an independent possessive form, meaning a form that can stand on its own without repeating the noun.

So:

• Die beker is ons s'n = The cup is ours.
• Die bord is hulle s'n = The plate is theirs.

The apostrophe shows that s'n is a shortened form.

It is not the same as the English possessive apostrophe in John's. In Afrikaans, s'n is a special possessive form meaning something like the one belonging to ....

So ons s'n literally has the idea of the one belonging to us, which English normally expresses as ours.

Yes, but that gives a different structure.

• ons beker = our cup
• hulle bord = their plate

Those are possessive words placed before a noun.

In your sentence, the noun has already been mentioned, so Afrikaans uses a stand-alone possessive:

• ons s'n = ours
• hulle s'n = theirs

So Die beker is ons s'n means The cup is ours, not The cup is our.

Because maar is a coordinating conjunction, like but in English.

After a coordinating conjunction, the next clause keeps normal main-clause word order:

..., maar die bord by die deur is hulle s'n.

So the verb is stays in its usual second position.

No, not here.

Afrikaans is simpler than languages like German or Dutch in this respect:

• die does not change for masculine, feminine, or neuter
• the nouns do not change form for case
• possession is shown with forms like ons s'n and hulle s'n, not with noun endings

That makes this sentence much more regular once you know the vocabulary.