Sy merk die kartondose met die swart merkpen.

Here Sy means she.

A useful thing to know is that sy can also mean his in other sentences, for example sy boek = his book. In this sentence, though, it comes before the verb merk, so it is clearly the subject pronoun she.

Afrikaans verbs usually do not change according to the subject in the present tense.

So you get:

• Ek merk = I mark
• Jy merk = you mark
• Sy merk = she marks
• Ons merk = we mark

Unlike English, Afrikaans does not add an -s for he/she/it in the present tense.

Merk means to mark, to label, or to tag, depending on context.

So in this sentence it suggests that she is putting some kind of identifying mark or label on the cardboard boxes. The exact English choice depends on the situation, but the Afrikaans verb itself is straightforwardly merk.

Because die means the, and Afrikaans uses it for both singular and plural nouns.

So:

• die kartondose = the cardboard boxes
• die swart merkpen = the black marker

English also uses the more than once when there are two separate noun phrases, so this is very natural.

In a sentence like this, die is just the definite article the.

Afrikaans die can sometimes look like English those in certain contexts, but here it simply means the. There is nothing demonstrative about it in this sentence.

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

So:

• karton = cardboard
• doos = box
• kartondoos = cardboard box
• kartondose = cardboard boxes

This is very normal in Afrikaans. English often writes similar combinations as two words, but Afrikaans usually joins them.

The singular is kartondoos.

So:

• kartondoos = cardboard box
• kartondose = cardboard boxes

The plural is not just doose. The word doos changes to dose in the plural, which is something learners simply need to get used to.

Here met means with, in the sense of using something.

So met die swart merkpen means with the black marker or using the black marker.

It does not mean that she is together with the marker in some personal sense. It shows the instrument used to do the action.

Because swart is an adjective, and in Afrikaans adjectives usually come before the noun, just as in English.

So:

• swart merkpen = black marker
• kartondose = cardboard boxes

That word order is completely normal.

Because swart is the normal form here.

Afrikaans adjectives sometimes take an extra -e before a noun, but not all of them do in all situations. Swart is commonly used just as swart in phrases like die swart merkpen.

So this is simply the correct standard form to learn here.

Yes, merkpen is one word.

It is a compound noun:

• merk = mark
• pen = pen

Together, merkpen means marker or marker pen. Depending on context, English might also say felt-tip marker or marking pen.

The sentence follows a very common Afrikaans pattern:

• Sy = subject
• merk = verb
• die kartondose = direct object
• met die swart merkpen = prepositional phrase showing the instrument

So the structure is basically:

Subject + Verb + Object + Prepositional phrase

That is very close to normal English word order here.

It describes the action.

It tells you how or with what she marks the boxes. So the black marker is the tool being used. It does not mean the boxes are black.

If the boxes were black, the wording would need to attach swart to kartondose, not to merkpen.

Not naturally in this context.

Although words related to merk can sometimes connect to noticing or perceiving in certain expressions, here merk with an object like die kartondose and the instrument phrase met die swart merkpen clearly means marking or labeling the boxes.

The phrase with the black marker strongly points to a physical marking action.