Wenn der Dübel richtig sitzt, können wir weiterschrauben.

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

So:

• Wenn der Dübel richtig sitzt = If the wall plug is seated properly

Structure:

• wenn = subordinating conjunction
• der Dübel = subject
• richtig = adverb
• sitzt = finite verb, placed at the end

This is one of the most important German word-order patterns to get used to.

Because the sentence starts with the subordinate clause:

• Wenn der Dübel richtig sitzt, ...

When a subordinate clause comes first, it takes up the first position in the sentence as a whole. That means the main clause must begin with the verb in second position:

• Wenn der Dübel richtig sitzt, können wir weiterschrauben.

This is normal German verb-second (V2) word order.

Compare:

• Wir können weiterschrauben, wenn der Dübel richtig sitzt.
• Wenn der Dübel richtig sitzt, können wir weiterschrauben.

Both are correct, but when the wenn-clause comes first, the main clause begins with können.

Here, der Dübel is the subject of the clause Wenn der Dübel richtig sitzt. Subjects are in the nominative case.

• Dübel is a masculine noun
• masculine nominative singular article = der

So:

• der Dübel = nominative singular
• literally: the wall plug / anchor

If it were in a different case, the article might change:

• nominative: der Dübel
• accusative: den Dübel
• dative: dem Dübel

But in this sentence, it is the thing that sits / is seated, so it is the subject.

Yes, but in German sitzen is also often used for things that are in place, properly seated, or fitted firmly.

So here, der Dübel sitzt does not mean the wall plug is literally sitting like a person. It means something like:

• the wall plug is seated
• the wall plug is in properly
• the wall plug is secure / fitting correctly

German uses sitzen very naturally for objects in this kind of context:

• Der Nagel sitzt fest. = The nail is firmly in place.
• Die Schraube sitzt locker. = The screw is loose.
• Der Dübel sitzt richtig. = The wall plug is seated properly.

This is a very common usage.

Because richtig is being used as an adverb, not as an adjective directly describing a noun.

It modifies the verb sitzt:

• sitzt richtig = sits/is seated properly

Adverbs do not take adjective endings in German.

Compare:

• der richtige Dübel = the correct wall plug
  Here richtige is an adjective describing Dübel.
• der Dübel sitzt richtig = the wall plug is seated properly
  Here richtig is an adverb, so it stays unchanged.

Weiterschrauben means to continue screwing or to keep screwing.

It is made up of:

• weiter = further, onward, continuing
• schrauben = to screw

Together, it expresses continuing the action.

In this sentence:

• können wir weiterschrauben = we can continue screwing

Depending on context, that could mean continuing to screw in a screw, continue fastening something, or keep working with the screw/driver.

Because in the infinitive, German often writes this kind of verb as one word.

Here it follows the modal verb können, so weiterschrauben appears in the infinitive at the end:

• können wir weiterschrauben

In other sentence forms, it can often behave like a separable verb:

• Wir schrauben weiter. = We continue screwing.
• Wir haben weitergeschraubt. = We continued screwing.

So a learner will often see:

• weiterschrauben as the infinitive
• schraubt weiter in a finite clause

That is normal.

Because German uses a comma to separate a subordinate clause from a main clause.

So in:

• Wenn der Dübel richtig sitzt, können wir weiterschrauben.

the comma marks the end of the wenn-clause and the beginning of the main clause.

This comma is required in standard German.

Wenn is the correct word here because it introduces a conditional clause:

• Wenn der Dübel richtig sitzt, ...
• If the wall plug is seated properly, ...

Compare:

• wenn = if / whenever
• als = when for a single event in the past
• wann = when in a question or indirect question

So:

• Wenn der Dübel richtig sitzt, können wir weiterschrauben. = correct
• Als der Dübel richtig saß... would mean something like When the wall plug was seated properly... in a past-time narrative, not a condition
• Wann der Dübel richtig sitzt... is not right here, because this is not a question about time

Because the object is understood from the situation.

In real German, especially in practical contexts, speakers often leave out things that are obvious from context. If you are already talking about installing something in a wall, weiterschrauben naturally means continuing the screwing task.

A fuller sentence could be something like:

• ...können wir die Schraube weiterschrauben.
• ...können wir weitermachen und die Schraube eindrehen.

But in the original sentence, the omitted object is completely natural.

Usually no, not if you want this exact meaning.

• der Dübel ist richtig means the wall plug is the right one / correct
• der Dübel sitzt richtig means the wall plug is seated properly / fits properly in place

So ist richtig talks about correctness in a general sense, while sitzt richtig talks about physical positioning or fit.

That distinction is important here.

Yes, it is a normal word in DIY, construction, and household contexts.

A Dübel is the piece you put into a wall so that a screw can hold securely. In English, depending on region and context, it may be called:

• wall plug
• rawlplug
• wall anchor

So the sentence sounds like something someone might say while mounting a shelf, cabinet, or bracket.

Yes. A very common alternative is:

• Wir können weiterschrauben, wenn der Dübel richtig sitzt.

This means the same thing.

The difference is mostly about emphasis and flow:

• Wenn der Dübel richtig sitzt, können wir weiterschrauben.
  Focuses first on the condition.
• Wir können weiterschrauben, wenn der Dübel richtig sitzt.
  Starts with the main point, then adds the condition.

Both are natural German.