Ons ondersoek nie soseer valdeure nie, maar skuifpanele.

Afrikaans often uses double negation. In a basic negative sentence, you usually get one nie after the verb phrase and another nie near the end of the clause.

Here, though, the sentence uses a fixed contrastive pattern:

nie soseer X nie, maar Y

This means not so much X, but Y.

So in:

Ons ondersoek nie soseer valdeure nie, maar skuifpanele.

the two nies are part of that structure, not just random repetition.

This is a very useful expression in Afrikaans. It means:

• not so much ... but ...
• rather ... than ...
• less ... and more ...

So the sentence is contrasting two things being investigated:

• valdeure = trapdoors
• skuifpanele = sliding panels

The idea is:

We are investigating not so much trapdoors, but sliding panels.

It does not mean trapdoors are completely excluded; it means the focus is more on the second item.

Yes, soseer is written as one word in this expression.

In nie soseer ... nie, maar ..., soseer helps express degree or emphasis, something like so much in English.

So:

• nie soseer X nie, maar Y = not so much X, but Y

You will most often meet soseer in this kind of contrastive phrase.

Here Ons means we.

It is the subject of the sentence:

• Ons = we
• ondersoek = investigate / examine

So:

• Ons ondersoek ... = We investigate ...

Afrikaans ons can also mean us in other contexts, but here the word order clearly shows it is the subject, so it means we.

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

Compare:

• Ek ondersoek = I investigate
• Jy ondersoek = you investigate
• Ons ondersoek = we investigate
• Hulle ondersoek = they investigate

The verb stays the same.

That is one of the nice simplifications of Afrikaans compared with English and Dutch.

Ondersoek means investigate, examine, or look into, depending on context.

It can be both:

• a verb: Ons ondersoek die saak. = We investigate the matter.
• a noun in other contexts: die ondersoek = the investigation

In your sentence, it is clearly a verb.

Yes. Both are compound nouns, which are very common in Afrikaans.

valdeure

• val = fall
• deure = doors
• together: trapdoors

skuifpanele

• skuif = slide / sliding
• panele = panels
• together: sliding panels

A helpful thing to notice: in Afrikaans compounds, the plural ending is usually on the last part:

• valdeur → valdeure
• skuifpaneel → skuifpanele

This is similar to English compounds like bookcase / bookcases.

Afrikaans often leaves out the article when speaking in a general sense.

So:

• Ons ondersoek valdeure
  = We investigate trapdoors / the trapdoors

Which English version sounds best depends on context. Afrikaans does not always force you to choose between trapdoors and the trapdoors as explicitly as English does.

If the context already makes the objects specific, English might prefer the. If the statement is more general, English might leave it out.

Maar usually means but.

In this sentence it introduces the contrast after nie soseer ... nie:

• nie soseer valdeure nie, maar skuifpanele
• not so much trapdoors, but sliding panels

So maar is the word that signals the correction or shift in focus.

Yes, often it can.

A natural English rendering could be:

• We are investigating sliding panels rather than trapdoors.
• We are investigating not so much trapdoors as sliding panels.

The exact English phrasing depends on style, but the Afrikaans structure is clearly making a contrast in emphasis.

Yes. The sentence follows normal main-clause word order:

• Ons = subject
• ondersoek = finite verb
• then the rest of the sentence

Afrikaans main clauses usually place the finite verb in the second position:

• Ons ondersoek ...

After that, the negative/contrastive expression is built around the objects:

• nie soseer valdeure nie, maar skuifpanele

So the sentence is grammatically very typical and natural.