Ons slaan die tent op.

On its own, slaan often does mean hit, strike, or beat.

But in this sentence, slaan ... op is a fixed verb combination: opslaan.
'n tent opslaan means to put up / pitch a tent.

So:

• slaan = hit / strike
• opslaan = put up, erect, pitch

This is very common in Afrikaans: the meaning of the whole verb can be different from the meaning of the base verb by itself.

Because opslaan is a separable verb.

In a normal main clause, the conjugated part goes earlier in the sentence, and the particle op moves to the end:

• Ons slaan die tent op.

This is similar to English phrasal verbs like set up, where the up can feel separate.

The full verb is still opslaan, but in this sentence it is split into:

• slaan ... op

The dictionary form is opslaan.

So if you look it up, you would usually find:

• opslaan = to put up / pitch / erect

In the sentence Ons slaan die tent op, the verb is split because of normal Afrikaans word order in a main clause.

Afrikaans verbs usually do not change form depending on the subject.

So:

• Ek slaan
• Jy slaan
• Hy slaan
• Ons slaan
• Hulle slaan

The verb stays slaan.

This is much simpler than English, which still has a small change in forms like I walk but he walks.

Ons means we.

It is the subject of the sentence—the people doing the action.

So:

• Ons = we
• slaan ... op = put up
• die tent = the tent

Die is the Afrikaans definite article, meaning the.

So:

• die tent = the tent

A useful thing to know is that Afrikaans uses die for all nouns, regardless of gender. There is no separate word like German der/die/das.

Also, die can be used for both singular and plural definite nouns:

• die tent = the tent
• die tente = the tents

Yes, depending on the meaning.

• Ons slaan die tent op. = We are putting up the tent.
  (a specific tent, already known in the situation)

• Ons slaan 'n tent op. = We are putting up a tent.
  (any tent / introducing it for the first time)

So the difference is the same basic difference as the versus a/an in English.

Grammatically, it is present tense.

• Ons slaan die tent op. = We put up / are putting up the tent.

But like in English, the present can sometimes refer to the near future if the context makes that clear. For example, while arriving at a campsite, it could naturally mean:

• We’re putting up the tent now
• We’re about to put up the tent

So the exact time depends on context.

Afrikaans often uses the simple present where English uses either:

• we put up
• we are putting up

So Ons slaan die tent op can cover both ideas, depending on context.

Afrikaans does have ways to show progressive meaning more explicitly, but very often the plain present tense is enough.

With separable verbs, the particle often joins the verb again in certain clause types.

For example:

• Ons slaan die tent op.
  = We are putting up the tent.

But:

• ... omdat ons die tent opslaan.
  = ... because we are putting up the tent.

So in a subordinate clause, you often see the full verb together again as opslaan.

That is a very common pattern in Afrikaans.

A rough English-friendly guide is:

• slaan ≈ slahn

The aa is a long vowel, not the short a sound in cat.

Very roughly:

• Ons ≈ ons / awns depending on accent
• slaan ≈ slahn
• die ≈ dee
• tent ≈ tent
• op ≈ op with a short o

Pronunciation varies a bit by accent, but that should get you close.

Yes. 'n tent opslaan is a normal, standard way to say to put up / pitch a tent.

So this is a very natural sentence:

• Ons slaan die tent op.

It is a useful expression to learn as a whole phrase:

• tent opslaan = pitch/put up a tent