Die leser maak die boek oop.

Because die is the definite article the in Afrikaans.

So:

• die leser = the reader
• die boek = the book

Afrikaans uses die before both nouns because both are definite.

No. That is one of the nice simple features of Afrikaans.

Die is used for:

• masculine nouns
• feminine nouns
• neuter nouns
• singular nouns
• plural nouns

So unlike languages such as German or Dutch, Afrikaans does not change the definite article for gender, and it does not use different forms like the singular vs plural.

Examples:

• die leser = the reader
• die lesers = the readers
• die boek = the book
• die boeke = the books

Yes. Leser is a noun meaning reader, and it is related to the verb lees meaning to read.

This is similar to English:

• read → reader

In Afrikaans:

• lees → leser

So leser is an agent noun: a person who does the action of reading.

Afrikaans very often expresses to open with oopmaak, literally make open.

So:

• maak ... oop = opens / is opening

This is very natural Afrikaans. English uses a single verb open, but Afrikaans commonly uses this two-part verb.

You can think of it as a fixed verbal expression:

• oopmaak = to open

Because oopmaak is a separable verb.

In a main clause, the finite verb comes earlier in the sentence, and the other part moves to the end:

• infinitive / dictionary form: oopmaak
• main clause: maak ... oop

So:

• Die leser maak die boek oop.

This is one of the most important patterns in Afrikaans word order.

Here it is part of the verb.

By itself, oop can mean open as an adjective:

• Die deur is oop. = The door is open.

But in your sentence, maak ... oop works together as the verbal idea open:

• Die leser maak die boek oop.

So although oop still has the idea of open, in this sentence it belongs to the separable verb oopmaak.

Very little. Afrikaans verbs are much simpler than English verbs.

With the present tense, the verb usually stays the same for all subjects:

• Ek maak = I open / make
• Jy maak = you open / make
• Hy maak = he opens / makes
• Ons maak = we open / make
• Hulle maak = they open / make

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

That is why you get:

• Die leser maak die boek oop.

not a special form like maaks.

The basic pattern is:

Subject + finite verb + object + separable part

So here:

• Die leser = subject
• maak = finite verb
• die boek = object
• oop = separable part

That gives:

• Die leser maak die boek oop.

This is a very common Afrikaans sentence structure.

Yes, often it can.

Afrikaans present tense is frequently broader than English present tense. Depending on context, maak ... oop can mean:

• opens
• is opening

If you specifically want to emphasize an ongoing action, Afrikaans can also use a progressive-type structure such as:

• Die leser is besig om die boek oop te maak.

But in ordinary language, the simple present often does the job.

You may hear open in some contexts, but maak ... oop is the more everyday, natural choice in many situations.

So for a learner, Die leser maak die boek oop is an excellent standard sentence.

In other words:

• oopmaak / maak ... oop = very common, natural Afrikaans
• open = possible in some contexts, but less basic for this kind of everyday example

In a yes/no question, Afrikaans usually puts the finite verb first:

• Maak die leser die boek oop?

That literally follows the pattern:

Verb + subject + object + separable part

So the separable part oop still stays at the end.

In a subordinate clause, the separable verb usually comes back together as one word at the end:

• Ek sien dat die leser die boek oopmaak.
• I see that the reader opens / is opening the book.

So compare:

• main clause: Die leser maak die boek oop.
• subordinate clause: ... dat die leser die boek oopmaak.

This is a very useful pattern to learn early.