As iemand vra oor die beker op die tafel, sal ek sê dat dit nie myne is nie.

Here as means if.

Afrikaans as can sometimes correspond to English if and sometimes to when, depending on context. In this sentence, it introduces a condition, so if is the natural understanding: if that situation happens, then I will say something.

It can cover both ideas.

In a sentence like this, iemand is an indefinite word meaning someone / anyone. Afrikaans does not make the same strict someone vs anyone distinction that English often does in conditional sentences. So in context, it means something like if someone asks or if anyone asks.

Because vra oor means ask about.

So:

• vra oor die beker = ask about the cup
• vra na die beker or sometimes vra vir die beker = ask for the cup

That is an important difference. In your sentence, the person is asking about the cup as a topic, not requesting the cup, so oor is the right preposition.

Because the sentence begins with a subordinate clause:

As iemand vra oor die beker op die tafel, ...

After that opening clause, Afrikaans normally uses a comma before the main clause:

..., sal ek sê ...

This is very similar to English punctuation in sentences like If someone asks..., I will say...

Because Afrikaans uses verb-second word order in the main clause.

The whole opening clause As iemand vra oor die beker op die tafel takes the first position in the sentence. Once that first position is filled, the finite verb of the main clause comes next:

• As iemand vra oor die beker op die tafel, sal ek sê ...

So the order is:

1. opening clause
2. finite verb = sal
3. subject = ek

If the sentence started directly with the subject, then you would get Ek sal sê ...

Sal usually corresponds to will.

Here it shows what the speaker will say in that situation:

• sal ek sê = I will say

In conditional sentences like this, it can also feel a bit like I would say in English, depending on tone and context, but the basic idea is future or expected response.

Dat means that and introduces the next clause:

• sal ek sê dat ... = I will say that ...

It is very common after verbs like sê.

Afrikaans can sometimes omit dat, especially in speech, but if you do that, the word order often changes. Compare:

• ... sê dat dit nie myne is nie
• ... sê dit is nie myne nie

So dat is not just a vocabulary item; it also affects the structure of the clause that follows.

Because after dat, Afrikaans uses subordinate-clause word order.

In that kind of clause, the verb tends to move toward the end:

• dat dit nie myne is nie

Without dat, you would normally use the more direct main-clause order:

• dit is nie myne nie

So the change in word order is caused by dat.

Because standard Afrikaans usually uses a double negative pattern.

The basic structure is:

• nie ... nie

So:

• Dit is nie myne nie = It is not mine
• dat dit nie myne is nie = that it is not mine

The first nie marks the negation, and the second nie closes it off at the end of the clause. This is one of the most famous features of Afrikaans grammar.

Because the clause is subordinate and the verb is comes later.

Compare these two patterns:

• main clause: Dit is nie myne nie
• dat-clause: dat dit nie myne is nie

In the main clause, is comes early, so nie follows it. In the dat-clause, the verb is pushed toward the end, so the negative structure shifts with it. The sentence is still the same idea, just with subordinate-clause word order.

Because my means my, but myne means mine.

Compare:

• my beker = my cup
• die beker is myne = the cup is mine

So in your sentence, the noun beker is not repeated. Instead, Afrikaans uses the possessive pronoun myne, which stands on its own.

Dit refers back to die beker.

Afrikaans often uses dit for things and objects instead of repeating the noun. So:

• die beker = the cup
• dit = it

That is why the sentence says dat dit nie myne is nie rather than repeating die beker again.

Yes, very often.

A very natural spoken version would be:

As iemand vra oor die beker op die tafel, sal ek sê dit is nie myne nie.

Notice what happens:

• with dat: dat dit nie myne is nie
• without dat: dit is nie myne nie

So yes, dat can be omitted, but the word order usually switches back to the more direct main-clause pattern.