Ek weet nie watter beker ek vir jou moet gee nie, want al twee lyk dieselfde.

Afrikaans normally uses double negation.

• The first nie usually comes after the finite verb or after the part being negated.
• The second nie usually comes near the end of the clause.

So:

• Ek weet nie = I do not know
• Ek weet nie ... nie = the full Afrikaans negative pattern

In this sentence:

• Ek weet nie watter beker ek vir jou moet gee nie
  literally follows the Afrikaans pattern: I know not which cup I for you must give not.

This is completely normal Afrikaans grammar, not extra emphasis.

Because this part is a subordinate clause introduced by watter.

In Afrikaans subordinate clauses, the verbs tend to move toward the end. With a modal verb such as moet, the usual pattern is:

• object(s) + moet
  • main verb

So:

• ek vir jou moet gee

and not normally:

• ek moet vir jou gee

Compare:

• Main clause: Ek moet vir jou die beker gee.
• Subordinate clause: ... watter beker ek vir jou moet gee.

This verb-final tendency is very common in Afrikaans subordinate clauses.

Watter means which.

So:

• watter beker = which cup

It is used when you are choosing from more than one possible item.

Examples:

• Watter boek wil jy hê? = Which book do you want?
• Ek weet nie watter pad ons moet vat nie. = I don’t know which road we should take.

In your sentence, the speaker does not know which cup to give because the two cups look the same.

Because watter itself functions as a determiner here, like English which.

English:

• which cup
  not which the cup

Afrikaans:

• watter beker
  not watter die beker

So watter already fills the slot where an article like ’n or die might otherwise appear.

Vir jou marks the indirect object: the person who receives something.

So:

• gee vir jou = give to you

In Afrikaans, vir is very commonly used before people/pronouns when they are the recipient.

Examples:

• Ek gee vir hom die boek. = I give him the book.
• Sy stuur vir ons ’n boodskap. = She sends us a message.

You may sometimes hear speech where vir is omitted in certain contexts, but vir jou is very natural and standard here.

Moet literally often corresponds to must, have to, or should, depending on context.

In this sentence, it does not necessarily sound as strong as English must can sound. Here it means something like:

• which cup I should give you
• which cup I’m supposed to give you
• which cup I need to give you

So moet is about what the speaker is required or expected to do in the situation.

Al twee means both.

So:

• al twee lyk dieselfde = both look the same

You can think of it as:

• al = all
• twee = two

Together, it means the two of them, both.

You may also come across altwee, which is another accepted form. Al twee is very common and clear.

Because want introduces a main clause-type structure, not the usual subordinate-clause word order.

So after want, Afrikaans keeps normal word order:

• want al twee lyk dieselfde

That is similar to English:

• because both look the same

Compare this with a subordinating conjunction such as omdat, which typically sends verbs to the end:

• ..., omdat al twee dieselfde lyk.

So:

• want → normal/main-clause order
• omdat → subordinate-clause order

That is an important distinction in Afrikaans.

Lyk dieselfde means look the same or seem the same.

It refers to appearance.

• Die twee bekers lyk dieselfde. = The two cups look the same.

Is dieselfde means are the same, which can suggest actual identity or sameness, not just appearance.

• Hulle is dieselfde. = They are the same.

In your sentence, lyk is used because the speaker is talking about how the cups appear, which is why it is hard to tell them apart.

Here dieselfde means the same or alike, and it is being used predicatively after lyk.

So:

• al twee lyk dieselfde = both look the same

Afrikaans often uses dieselfde this way, just like English can say They look the same.

You can also use it before a noun:

• dieselfde beker = the same cup
• dieselfde dag = the same day

So it can appear:

• before a noun, or
• on its own after a verb like is or lyk

Beker often means cup, but depending on context it can also mean things like:

• a drinking cup
• a mug
• a goblet
• a trophy/cup in sports

In this sentence, because of gee vir jou and the general context, it most naturally means an ordinary cup.

Afrikaans often relies on context for this kind of word.

Yes. A few natural alternatives are possible, depending on style.

For example:

• Ek weet nie watter beker ek vir jou moet gee nie, want hulle lyk al twee dieselfde.
• Ek weet nie watter een ek vir jou moet gee nie, want al twee lyk dieselfde.

These mean roughly the same thing.

The original sentence is already correct and natural. The alternatives just make different parts more explicit:

• hulle = they
• watter een = which one

Yes, several things:

1. Double negation

   • Ek weet nie ... nie

2. Subordinate clause word order

   • ... watter beker ek vir jou moet gee
   • the verbs move toward the end

3. Use of vir with a recipient

   • vir jou

4. Main-clause order after want

   • want al twee lyk dieselfde

5. Afrikaans often uses forms that are more compact than English

   • al twee = both
   • dieselfde = the same

So this one sentence actually shows several very typical Afrikaans grammar patterns.