Ons toets nie soseer die stoompyp nie, maar die stoomklep.

Because standard Afrikaans negation is usually double-marked.

In many ordinary negative sentences, Afrikaans uses:

• a first nie somewhere after the verb area, and
• a second nie near the end of the clause.

So in:

Ons toets nie soseer die stoompyp nie, maar die stoomklep.

the first nie starts the negative idea, and the second nie closes it.

This is normal Afrikaans grammar. It is not the same as an English-style double negative like I didn't do nothing.

This is a very common contrast pattern in Afrikaans.

nie soseer X nie, maar Y means:

• not so much X, but Y
• not really X, but rather Y
• less X than Y

So the sentence is contrasting two things:

• die stoompyp
• die stoomklep

The idea is that the second item is the more important or more accurate focus.

Here soseer means something like so much, particularly, or to that extent.

But learners should really understand it as part of the whole expression:

nie soseer ... nie, maar ...

rather than trying to translate soseer word-for-word every time.

So:

• nie soseer die stoompyp nie, maar die stoomklep means
• not so much the steam pipe, but the steam valve

Because the sentence is using the set contrast pattern nie soseer ... nie.

So the language naturally groups it like this:

• Ons toets
• nie soseer die stoompyp nie
• maar die stoomklep

The negative is attached to the idea not so much X, not just to the verb by itself.

If you moved things around, the sentence would sound less natural or would change the emphasis.

Maar usually means but.

In this structure, though, it often feels closer to:

• but rather
• but instead

So after nie soseer ... nie, maar introduces the item that is the better description, stronger focus, or real point of contrast.

Die is the definite article in Afrikaans, meaning the.

Afrikaans uses die for:

• singular nouns
• plural nouns
• all grammatical genders

So both nouns here take die:

• die stoompyp
• die stoomklep

Afrikaans repeats the article before each noun phrase, just as English normally does in the steam pipe ... the steam valve.

Because Afrikaans, like Dutch and German, very often forms compound nouns as one word.

So:

• stoom
  • pyp = stoompyp
• stoom
  • klep = stoomklep

The last part is the main noun:

• a stoompyp is a kind of pyp
• a stoomklep is a kind of klep

This is very typical Afrikaans spelling.

Usually, no. Afrikaans verbs do not change much for person in the present tense.

So you get:

• ek toets
• jy toets
• hy/sy toets
• ons toets
• julle toets
• hulle toets

That is much simpler than English, where you have forms like I test but he tests.

It would be similar, but not exactly the same.

• Ons toets nie die stoompyp nie, maar die stoomklep
  = We are not testing the steam pipe, but the steam valve.

This sounds more direct and absolute.

• Ons toets nie soseer die stoompyp nie, maar die stoomklep
  = We are testing not so much the steam pipe as the steam valve.

This version is softer and more nuanced. It suggests:

• the steam pipe is not the main focus, or
• the steam valve is the more accurate thing to mention.

So soseer adds a contrast of emphasis, not just plain replacement.

In this pattern, the second nie comes at the end of the negated part of the clause.

Here the negated section is:

nie soseer die stoompyp nie

Then the contrasting phrase comes after it:

maar die stoomklep

That is why the second nie appears before the comma and before maar.

Toets usually means test, check, examine, or assess, depending on context.

For example, it can be used for:

• testing equipment
• checking a system
• assessing knowledge in school
• trying whether something works

So in this sentence, toets could mean something like test or check, depending on the situation.

Yes. Ons toets is present tense.

In Afrikaans, the simple present can cover several English ideas, depending on context, such as:

• we test
• we are testing
• we do test

So without extra context, Ons toets is simply present tense, and English translation depends on what sounds best in the situation.