Danach schrauben wir das Regal fest, obwohl ein Dübel zuerst nicht richtig sitzt.

Because festschrauben is a separable verb.

• In the infinitive, you write it as one word: festschrauben
• In a normal main clause with a conjugated verb, the prefix separates: Wir schrauben das Regal fest

So in your sentence:

• infinitive idea: das Regal festschrauben
• actual main clause: Danach schrauben wir das Regal fest

This is very common in German:

• anrufen → Ich rufe dich an
• aufmachen → Er macht das Fenster auf
• festschrauben → Wir schrauben das Regal fest

German main clauses usually follow the verb-second rule.

That means the conjugated verb must be in the second position. If you put Danach first, the verb still has to stay second, so the subject moves after it:

• Wir schrauben danach das Regal fest
• Danach schrauben wir das Regal fest

Both are possible, but when Danach is placed first for emphasis or flow, schrauben must come immediately after it.

So the pattern is:

• first element: Danach
• second element: schrauben
• then subject: wir

Because obwohl introduces a subordinate clause, and in German subordinate clauses the conjugated verb usually goes to the end.

So:

• main clause: Danach schrauben wir das Regal fest
• subordinate clause: obwohl ein Dübel zuerst nicht richtig sitzt

This verb-final pattern happens with many subordinating conjunctions, such as:

• weil
• dass
• obwohl
• wenn

Examples:

• weil er müde ist
• dass sie heute kommt
• obwohl ein Dübel nicht richtig sitzt

Das Regal is in the accusative case because it is the direct object of schrauben ... fest.

It is the thing being screwed/fixed into place.

Here:

• wir = subject
• schrauben = verb
• das Regal = direct object

For das, nominative and accusative look the same, so the form does not change:

• nominative: das Regal
• accusative: das Regal

Even though the form looks the same, its role in the sentence is accusative.

Because ein Dübel is the subject of the subordinate clause, so it is in the nominative case.

The clause is:

obwohl ein Dübel zuerst nicht richtig sitzt

Here, the dowel is the thing that sits / is positioned / fits. It is not a direct object.

Since Dübel is masculine:

• nominative: ein Dübel
• accusative: einen Dübel

You would use einen Dübel only if it were the object of a verb, for example:

• Ich setze einen Dübel in die Wand.

But in your sentence, ein Dübel is the subject of sitzt.

In German, sitzen is often used more broadly than English sit.

Here, ein Dübel sitzt nicht richtig means something like:

• it is not seated properly
• it is not positioned correctly
• it does not fit securely

German often uses sitzen for things that are fixed, fitted, or resting in the right place:

• Die Schraube sitzt locker.
• Die Brille sitzt gut.
• Der Stecker sitzt fest.

So this is very natural German, even though English would usually choose a different verb.

Both can often be translated as then, but they are not always identical.

In this sentence, danach means after that or afterwards, and it clearly links this action to a previous step.

• Danach schrauben wir das Regal fest = After that, we screw the shelf unit firmly in place

Dann can also mean then, but it is sometimes a little more general and less explicitly tied to a specific earlier action.

So here, danach works well because the sentence sounds like part of a sequence of instructions or events.

Zuerst means at first or initially.

So:

ein Dübel zuerst nicht richtig sitzt

means that at the beginning, the dowel is not fitting or sitting properly.

The idea is:

• first, there is a problem
• later, the process still continues or the problem gets resolved

Nicht richtig means not correctly / not properly.

Together, zuerst nicht richtig gives the sense of:

• not properly at first
• initially not in the right position

Yes. In German, a subordinate clause introduced by a conjunction like obwohl must be separated by a comma.

So this is correct:

Danach schrauben wir das Regal fest, obwohl ein Dübel zuerst nicht richtig sitzt.

This is a standard rule with subordinate clauses introduced by words such as:

• weil
• dass
• wenn
• obwohl

Yes, but only in forms where the verb is not separated.

For example:

• infinitive: das Regal festschrauben
• with a modal: Wir müssen das Regal festschrauben
• perfect participle: Wir haben das Regal festgeschraubt

But in a normal present-tense main clause, the prefix separates:

• Wir schrauben das Regal fest

So both spellings are correct, depending on the grammar of the sentence.

German often uses the present tense to describe:

• instructions
• routines
• narrative steps
• actions in a sequence

So Danach schrauben wir das Regal fest can sound like:

• a description of what we do next
• a step in assembly instructions
• a narrative present

English also sometimes does this, especially in instructions:

• Next, we tighten the screws
• Then we attach the shelf

So the present tense here is completely natural.