Wanneer ek nies, bring my suster vir my water en sê ek moet by die huis bly.

It can be understood as either, depending on context.

In this sentence, because the verbs are in the present tense, wanneer ek nies often has a general or repeated sense: when/whenever I sneeze.

So the sentence can sound like a habitual situation, not just one single event.

This is because Afrikaans is a verb-second language in main clauses.

The opening time clause Wanneer ek nies takes the first position in the sentence. After that, the finite verb of the main clause must come next:

• Wanneer ek nies, bring my suster vir my water...

If there were no opening clause, the normal order would be:

• My suster bring vir my water...

So the word order changes because the sentence begins with a time expression.

Afrikaans verbs usually do not change according to the subject the way English verbs do.

Compare:

• Ek bring = I bring
• Jy bring = you bring
• My suster bring = my sister brings

English adds -s in she brings, but Afrikaans keeps the same form: bring.

The same is true for sê:

• Ek sê
• Sy sê

So Afrikaans verb conjugation is much simpler than English here.

Because my can be both:

• a possessive determiner: my suster = my sister
• an object pronoun after vir: vir my = to me / for me

So the two my words are not doing the same job.

You can think of it like this:

• my suster → possession
• vir my → recipient/object

This is very common in Afrikaans.

Here vir marks the person receiving the thing.

So:

• bring vir my water = bring me water

Literally, it is close to bring water to/for me, though in natural English we usually just say bring me water.

With people, Afrikaans very often uses vir like this:

• Hy gee vir my brood. = He gives me bread.
• Sy stuur vir hom ’n boodskap. = She sends him a message.

So in your sentence, vir my shows who receives the water.

Because Wanneer ek nies is an introductory subordinate clause.

Afrikaans normally places a comma between that opening clause and the main clause:

• Wanneer ek nies, bring my suster...

This is very similar to English:

• When I sneeze, my sister...

So the comma is standard punctuation here.

Afrikaans often leaves out dat when it means that.

So both of these are possible:

• ...en sê ek moet by die huis bly.
• ...en sê dat ek moet by die huis bly.

Both mean the same thing. Leaving out dat is very common in everyday Afrikaans.

Because moet is a modal verb, and modal verbs in Afrikaans are followed by a bare infinitive placed later in the clause.

So:

• ek moet ... bly = I must stay

Not:

• ek moet ... te bly

After modal verbs such as kan, moet, wil, sal, Afrikaans does not use te.

Examples:

• Ek kan swem. = I can swim.
• Sy wil gaan. = She wants to go.
• Ons moet bly. = We must stay.

So in your sentence, bly goes at the end as the main action after moet.

Literally, yes, the words are:

• by = at/by
• die huis = the house

But as an expression, by die huis bly usually means stay at home.

So this is a normal idiomatic way to say at home, not necessarily next to the house.

Another Afrikaans way to express at home is tuis, but by die huis is very common and natural.

Because water here is being used as a mass noun in a general sense.

Afrikaans often leaves out the article with substances or uncountable nouns when it just means some water:

• bring vir my water = bring me water

If you wanted to refer to specific water, you could use die:

• bring vir my die water = bring me the water

So the lack of an article here is completely normal.

Most naturally, it sounds like a repeated or usual situation:

• Whenever/when I sneeze, my sister brings me water and says I must stay at home.

That feeling comes from:

• wanneer
• the present tense
• the general style of the sentence

So a learner should usually understand it as something habitual, unless the wider context makes it a one-time event.