Ich merke das Buch vor.

Because vormerken is a separable verb.

The full verb is vormerken, but in a normal main clause, the prefix vor- separates and moves to the end:

• Ich merke das Buch vor.

In other forms, it stays together:

• infinitive: das Buch vormerken
• perfect participle: das Buch vorgemerkt
• subordinate clause: ..., weil ich das Buch vormerke

So vor is not acting as a preposition here. It is part of the verb.

Not really. In this sentence, you should treat vormerken as a separate vocabulary item.

• merken by itself often means to notice
• sich etwas merken means to remember something
• etwas vormerken means to reserve something, put it on hold, or note it down in advance

So you should not translate this sentence word-for-word as something like I notice the book ahead of time. The combined verb has its own meaning.

Das Buch is the direct object, so it is in the accusative case.

The verb vormerken takes an object: you reserve something.

Here, Buch is a neuter noun, and for neuter singular, the nominative and accusative forms are both das Buch. So the form does not change.

For comparison:

• nominative: das Buch ist interessant
• accusative: ich merke das Buch vor

If the noun were masculine, you would see the case more clearly:

• Ich merke den Film vor.

Yes, it can be natural, especially in contexts like:

• a library
• a waiting list
• a course or appointment system
• reserving something for later

With ein Buch, vormerken often suggests putting the book on hold or requesting it in advance, especially if it is not available right now.

Depending on the situation, German speakers might also use reservieren, but vormerken is very common when something is being noted for later availability rather than simply booked like a hotel room or restaurant table.

Because that would mix two different constructions.

There are two different verbs/patterns here:

1. sich etwas merken
   = to remember something

   • Ich merke mir das Buch.
     = I will remember the book.

2. etwas vormerken
   = to reserve / put something on hold / note something down in advance

   • Ich merke das Buch vor.
     = I reserve the book / put the book on hold.

So mir does not belong in the sentence unless you mean something else.

Here are some useful forms of vormerken:

• present: Ich merke das Buch vor.
• perfect: Ich habe das Buch vorgemerkt.
• infinitive with a modal: Ich möchte das Buch vormerken.
• subordinate clause: ..., weil ich das Buch vormerke.

This is a very common pattern with separable verbs:

• main clause: split
• infinitive / participle / subordinate clause: together

Yes. You can add that information, often with für + accusative.

For example:

• Ich merke das Buch für meine Freundin vor.
• Ich merke das Buch für mich vor.

That means you are reserving the book for someone.

So the basic sentence is fine on its own, but German often adds extra context if needed.

The dictionary form is vormerken.

When you look it up, do not search for just merken if you want this meaning. The sentence uses the separable verb vormerken, and that is the form you should learn.

A good vocabulary note would be:

• vormerken = to reserve, to put on hold, to note down in advance

That will help you recognize it in forms like:

• er merkt vor
• ich habe vorgemerkt
• wir wollen vormerken