Ich montiere den Sattel ab.

Because the full verb is abmontieren, and ab- is a separable prefix.

In a normal main clause, German usually puts the conjugated part of the verb in position 2 and sends the separable prefix to the end:

• Ich montiere den Sattel ab.

So:

• abmontieren = infinitive
• montiere = conjugated verb
• ab = separated prefix

This is very common in German:

• Ich stehe auf. = I get up.
• Er kauft ein. = He shops / buys groceries.

The base verb is abmontieren.

It is made up of:

• montieren = to mount, assemble, install
• ab- = a prefix that often gives the idea of off, away, or removal

So abmontieren means something like:

• to remove
• to unmount
• to take off by unscrewing or detaching

Because Sattel is the direct object of the verb, so it takes the accusative case.

Sattel is a masculine noun:

• nominative: der Sattel
• accusative: den Sattel

Here, the person is doing something to the saddle, so the saddle is the thing being acted on:

• Ich montiere den Sattel ab.

That is why German uses den, not der.

Ask yourself: what is being removed?

• Ich = the subject, the person doing the action
• montiere ... ab = the action
• den Sattel = the thing being removed

So den Sattel is the direct object.

In English, word order mostly shows this. In German, word order helps too, but the article also makes it clear:

• den marks masculine accusative

No. Sattel can mean a saddle in the traditional sense, but it is also commonly used for a bicycle saddle or similar seat.

So in many everyday contexts, especially with verbs like abmontieren, der Sattel is likely a bike saddle or seat.

The exact meaning depends on context.

Because German main clauses follow the verb-second rule, often called V2.

That means the finite verb goes in the second position:

• Ich montiere den Sattel ab.

Here:

1. Ich
2. montiere

Even if something else comes first, the verb still stays second:

• Heute montiere ich den Sattel ab.
• Den Sattel montiere ich heute ab.

So the key rule is not subject first, but finite verb second.

Not in a normal main clause. That would usually be incorrect.

In a main clause, the separable prefix must split off:

• correct: Ich montiere den Sattel ab.
• incorrect: Ich abmontiere den Sattel.

But the prefix stays attached in some other structures, for example:

• ... weil ich den Sattel abmontiere.
• Ich habe den Sattel abmontiert.
• den Sattel abmontieren

So whether the prefix separates depends on the grammar of the sentence.

In the perfect tense, separable verbs usually use ge between the prefix and the verb stem.

So:

• infinitive: abmontieren
• past participle: abmontiert

Example:

• Ich habe den Sattel abmontiert. = I removed the saddle.

Structure:

• ab-
  • montiert
• with haben as the auxiliary

Not exactly. On its own, montieren usually means to mount, to assemble, or to install. The idea of removal comes from the prefix ab-.

So:

• montieren = to install / mount
• abmontieren = to remove / unmount / take off

This is why you should learn the whole verb abmontieren, not just assume it means the same as montieren.

Yes, people do say it, especially in technical or practical contexts.

Depending on the situation, German speakers might also use:

• abnehmen = take off
• entfernen = remove
• ausbauen = remove by taking out
• demontieren = dismantle / remove

But den Sattel abmontieren sounds natural when the saddle is attached and you are removing it mechanically.

In abmontieren, the stress is normally on -tier-:

• ab-mon-TIE-ren

In the sentence, the separated prefix ab is still spoken clearly at the end:

• Ich montiere den Sattel ab.

In natural speech, the final ab often gets noticeable emphasis because it completes the meaning of the separable verb.