Niemand kan weet hoekom die bottel reeds leeg is nie.

Because Afrikaans normally uses negative concord: a negative idea is often marked twice.

In this sentence:

• niemand = nobody / no one
• final nie closes the negative statement

So Afrikaans says, in effect, Nobody can know ... not.

That may feel redundant to an English speaker, but it is normal Afrikaans grammar.

Compare:

• Niemand verstaan dit nie. = Nobody understands it.
• Niks help nie. = Nothing helps.

With words like niemand, niks, and geen, you usually still get the final nie.

Because that nie belongs to the main clause, and in Afrikaans the second negative marker usually comes near the end of the whole clause.

Here the structure is:

• Niemand kan weet = main clause
• hoekom die bottel reeds leeg is = embedded clause
• nie = closes the negative main clause

So Afrikaans keeps the whole idea together and then ends the negative with nie:

• Niemand kan weet hoekom die bottel reeds leeg is nie.

This is very common when a main clause contains a subordinate clause.

Because this is an embedded question, not a direct question.

Direct question:

• Hoekom is die bottel reeds leeg?
• Why is the bottle already empty?

Embedded question:

• Niemand kan weet hoekom die bottel reeds leeg is nie.
• Nobody can know why the bottle is already empty.

In embedded questions, Afrikaans does not use the normal direct-question inversion. The word order becomes more like a subordinate clause, with the finite verb later in the clause.

Because in this subordinate clause, the finite verb goes toward the end.

So you get:

• hoekom die bottel reeds leeg is

not:

• hoekom die bottel is reeds leeg

Here:

• die bottel = subject
• reeds = adverb
• leeg = predicate adjective
• is = finite verb, placed at the end of the subordinate clause

This is a very important Afrikaans pattern:

• Ek weet dat hy moeg is.
• Sy vra hoekom die deur oop is.

Yes. Reeds means already.

So:

• die bottel reeds leeg is = the bottle is already empty

However, learners should know that reeds can sound a bit more formal or written than al, which is very common in everyday speech.

You may also hear:

• die bottel is al leeg
• the bottle is already empty

So reeds is correct, but al is often more conversational.

Kan is a modal verb meaning can / be able to.

In Afrikaans, when a modal verb is used, the next verb usually stays in the infinitive/base form:

• kan weet = can know
• moet gaan = must go
• wil sien = want to see

So:

• Niemand kan weet ...
• Nobody can know ...

This is similar to English in meaning, although Afrikaans word order is often a bit different.

Afrikaans usually distinguishes between:

• weet = to know a fact, answer, piece of information
• ken = to know a person, place, or thing through familiarity

Here the sentence is about knowing why something happened, so it uses weet.

Examples:

• Ek weet hoekom. = I know why.
• Sy weet die antwoord. = She knows the answer.
• Ek ken hom. = I know him.
• Ken jy Kaapstad? = Do you know Cape Town? / Are you familiar with Cape Town?

So weet is the right choice because this is about knowledge of an explanation.

It is understood as singular in meaning: no one / nobody.

But Afrikaans verbs do not change much for person and number, so you do not see a big verb-agreement difference the way you might in some other languages.

So:

• Niemand kan ... = Nobody can ...

You can think of niemand as a singular indefinite subject.

Yes. Both hoekom and waarom mean why.

So you could also say:

• Niemand kan weet waarom die bottel reeds leeg is nie.

In many situations:

• hoekom sounds more common and conversational
• waarom can sound a bit more formal or careful

Both are correct here.

Yes. Die is the Afrikaans definite article, and here it means the:

• die bottel = the bottle

Afrikaans does not have grammatical gender the way Dutch or German do, so die is used very widely for the.

One important note for English speakers: die is not pronounced like English die. It is pronounced more like dee.

Because this is a normal statement, and Afrikaans main clauses usually follow a verb-second pattern.

If the subject comes first, the finite verb comes second:

• Niemand
  • kan
    • weet ...

That is the standard declarative order.

You would only move things around for special emphasis or in questions. So Niemand kan weet ... is the natural neutral form.