Ons slaan die tent op, tensy die staanplek nat is.

Yes, on its own slaan often means hit / strike. But in this sentence it is part of the fixed verb opslaan, which can mean put up, set up, or pitch depending on the context.

So:

• slaan = hit / strike
• opslaan = put up / set up / pitch

With tent, die tent opslaan means to pitch the tent.

This is a very common thing in Afrikaans: a verb plus a small word like op, af, uit, etc. can create a new meaning.

Because opslaan is a separable verb in Afrikaans.

In a main clause, the main verb is split:

• Ons slaan die tent op.

But in forms where the verb stays together, it is written as one word:

• om die tent op te slaan = to pitch the tent
• Ons wil die tent opslaan = We want to pitch the tent
• Hy het die tent opgeslaan = He pitched the tent

So in your sentence, slaan appears in the normal verb position, and op moves to the end of that clause.

Tensy means unless.

It introduces an exception:

• Ons slaan die tent op, tensy die staanplek nat is. = We pitch the tent, unless the pitch/site is wet.

A useful way to understand it is:

• We will do X, except if Y happens.

So here:

• X = pitch the tent
• Y = the site is wet

Because tensy introduces a subordinate clause, and in Afrikaans subordinate clauses usually send the conjugated verb to the end.

Compare:

• Main clause: Die staanplek is nat.
• After tensy: tensy die staanplek nat is

This is very similar to what happens after other subordinating conjunctions such as:

• omdat = because
• as = if / when
• terwyl = while
• dat = that

So the end position of is is completely normal here.

Staanplek literally breaks down as:

• staan = stand
• plek = place

So staanplek is literally standing place, but in context it means something like:

• camping spot
• pitch
• site
• place to put the tent

If the sentence is about camping, pitch or campsite spot is usually the most natural English idea.

Correct: Afrikaans has just one common definite article, die, for all nouns.

So unlike languages that change the for gender or number, Afrikaans keeps it simple:

• die tent = the tent
• die staanplek = the site/pitch
• die man = the man
• die vrou = the woman
• die kinders = the children

So die does not change for masculine, feminine, neuter, or plural.

In Afrikaans, the present tense is often used for a future meaning when the context makes it clear.

So:

• Ons slaan die tent op can mean We pitch the tent or We’re going to pitch the tent

In a camping situation, it naturally sounds future-like: We’ll pitch the tent.

If you want to make the future more explicit, you can use sal:

• Ons sal die tent opslaan, tensy die staanplek nat is.

That means essentially the same thing, but it makes the future more obvious.

In a normal main clause, it would be:

• Die staanplek is nat.

But after tensy, the clause becomes subordinate, so the verb goes to the end:

• tensy die staanplek nat is

So nat stays as the adjective, and is moves to the final position because of the clause type.

Not in this exact sentence.

In a main clause with normal word order, the separable verb splits:

• Ons slaan die tent op.

If you wrote Ons opslaan die tent, that would be wrong.

But the one-word form opslaan does appear in other structures, for example:

• Ons wil die tent opslaan.
• om die tent op te slaan
• Die tent is opgeslaan.

So whether it is split or joined depends on the grammar of the sentence.

It is a very natural way to say it, but there are alternatives depending on style and region.

Some possibilities are:

• Ons slaan die tent op = We pitch the tent
• Ons sit die tent op = We put up the tent
• Ons rig die tent op = We erect the tent

For everyday camping language, die tent opslaan is very common and idiomatic.

Afrikaans usually uses double negation with nie ... nie.

So:

• Ons slaan nie die tent op nie, tensy die staanplek nat is.

That means We are not pitching the tent, unless the site is wet.

If you wanted to negate the second part instead, you could say:

• Ons slaan die tent op, tensy die staanplek nie nat is nie.

That would mean We pitch the tent, unless the site is not wet, which is logically awkward, but it shows how the negation works.

So a key point for learners is that standard Afrikaans normally uses two nies in a negative clause.