Die koper klop aan die deur.

Because die is the Afrikaans definite article, meaning the. It is used for:

• singular nouns
• plural nouns
• all genders

So Afrikaans does not change the word for the the way some languages do. In this sentence:

• die koper = the buyer
• die deur = the door

Usually, yes, koper means buyer or purchaser.

It is related to the verb koop = to buy, and -er is a common ending for a person who does something, like English -er in buyer.

One thing to watch out for: koper can also mean copper in other contexts. So context matters. In this sentence, since someone is knocking at the door, the intended meaning is clearly the person: the buyer.

In Afrikaans, verbs usually do not change form in the present tense the way English verbs do.

So you get:

• Ek klop = I knock
• Jy klop = you knock
• Hy klop = he knocks
• Ons klop = we knock

The verb stays klop. This is one of the nice simplifications in Afrikaans.

Here, klop aan die deur means to knock at/on the door.

The verb klop by itself can mean things like:

• knock
• beat
• strike
• thump

But in the expression aan die deur klop, it specifically refers to knocking at a door. So learners often remember it as a fixed phrase:

• aan die deur klop = to knock on the door

Because prepositions do not always match English exactly.

English says:

• knock on the door

Afrikaans normally says:

• aan die deur klop

So even though English uses on, Afrikaans uses aan in this expression. It is best learned as a set phrase rather than translated word-for-word.

Yes, this is normal Afrikaans main-clause word order.

The pattern is:

• subject: Die koper
• verb: klop
• rest of the sentence: aan die deur

Afrikaans usually follows the verb-second rule in main clauses. Since the subject comes first here, the verb comes next:

• Die koper klop aan die deur.

If you move another element to the front, the verb still stays in second position:

• Aan die deur klop die koper.

That version is more marked or literary, but grammatically possible.

No. Deur can be both:

• a noun: door
• a preposition: through (and sometimes by in passive constructions)

In this sentence, it is clearly the noun because it has the article die before it:

• die deur = the door

Compare:

• Hy loop deur die park. = He walks through the park.

So the article helps you see that deur here means door, not through.

Afrikaans normally uses a double nie in negative sentences.

So:

• Die koper klop aan die deur. becomes
• Die koper klop nie aan die deur nie.

That is a very important Afrikaans pattern:

• first nie after the verb or verbal element
• second nie at the end of the clause

A common past-tense form would be:

• Die koper het aan die deur geklop.
  = The buyer knocked at the door.

This uses the usual Afrikaans perfect construction:

• het
  • past participle

So:

• klop → geklop

Notice that the word order changes in the past tense:

• Die koper het aan die deur geklop.

Normally, no, not if you mean knocks on the door.

• Die koper klop aan die deur = The buyer knocks on the door.

If you say klop die deur, it sounds unnatural for this meaning and may suggest physically hitting or beating the door in a more direct way. For the ordinary idea of knocking at a door, use:

• aan die deur klop