Ons som die hoofstuk op.

Because this is a separable verb in Afrikaans: opsom, meaning to summarize / sum up.

In a main clause, the finite verb goes into the normal verb position, and the particle op moves to the end:

• Ons som die hoofstuk op.

But in some other structures, it stays together:

• om die hoofstuk op te som = to summarize the chapter
• dat ons die hoofstuk opsom = that we summarize the chapter

So the split is normal and important.

This happens because of Afrikaans word order with separable verbs.

The basic pattern here is:

• Ons = subject
• som = finite verb
• die hoofstuk = object
• op = separable particle

So op goes to the end in a main clause:

• Ons som die hoofstuk op.

This is similar to how some German and Dutch verbs behave.

Yes, som can exist by itself, but the meaning changes.

• som on its own usually relates to adding up or calculating
• som ... op / opsom means summarize

So:

• Ons som die getalle. = We add up the numbers.
• Ons som die hoofstuk op. = We summarize the chapter.

That little op changes the meaning a lot.

Because Afrikaans verbs usually do not change much for person or number in the present tense.

So you get:

• Ek som op
• Jy som op
• Hy som op
• Ons som op

Unlike English, where you have I summarize but he summarizes, Afrikaans normally keeps the verb form the same.

Here, ons means we.

It is the normal first-person plural subject pronoun in Afrikaans. Depending on context, it can be:

• inclusive: you and I and maybe others
• exclusive: I and others, but not you

Usually context tells you which one is meant.

Also, ons can sometimes mean us in other sentence roles, but here it is clearly the subject, so it means we.

die is the definite article in Afrikaans, meaning the.

So:

• die hoofstuk = the chapter

Afrikaans does not have different forms like the / a / an based on gender. For the indefinite article, you would use:

• 'n hoofstuk = a chapter

So the sentence uses die because it refers to a specific chapter.

A very common past form in Afrikaans is the perfect, using het plus a past participle.

For opsom, the participle is opgesom:

• Ons het die hoofstuk opgesom. = We summarized the chapter.

Notice that with the participle, the verb is no longer split the same way as in the present tense main clause. It becomes opgesom.

So compare:

• Ons som die hoofstuk op. = We summarize the chapter.
• Ons het die hoofstuk opgesom. = We summarized the chapter.

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

So the negative sentence is:

• Ons som die hoofstuk nie op nie.

Pattern:

• subject
• verb
• object
• nie
• final particle of the separable verb
• final nie

This double nie is a very common feature of standard Afrikaans.