Door de retourzending werd de levertijd langer.

Here door means because of or due to.

In Dutch, door can mean different things depending on context:

• through: door het raam = through the window
• by (in a passive sentence): door Jan = by Jan
• because of / due to: door de regen = because of the rain

In Door de retourzending werd de levertijd langer, it expresses a cause: because of the return shipment, the delivery time became longer.

So here it is not literal movement, and it is not the agent of a passive action.

Dutch main clauses follow the V2 rule: the finite verb must be in the second position.

If you put Door de retourzending first for emphasis, the verb must come next:

• Door de retourzending werd de levertijd langer.

If you start with the subject instead, you get:

• De levertijd werd door de retourzending langer.

Both are grammatical. Starting with Door de retourzending puts more focus on the cause.

Because the sentence begins with a different element: Door de retourzending.

In Dutch main clauses, once something other than the subject comes first, the finite verb still has to stay in second position, and the subject moves after it.

So:

• normal order: De levertijd werd langer.
• with a fronted phrase: Door de retourzending werd de levertijd langer.

This is called inversion.

Werd is the past tense of worden, which often means to become.

So:

• werd langer = became longer
• was langer = was longer

That is an important difference:

• De levertijd werd langer = the delivery time increased / became longer
• De levertijd was langer = the delivery time already was longer

The Dutch sentence describes a change, so werd is the natural choice.

Not really, even though werd can also be used in passive constructions.

Compare:

• Het pakket werd verstuurd. = The package was sent.
  Here werd is a passive auxiliary.

But in your sentence:

• De levertijd werd langer.

Here langer is an adjective, not a past participle. So werd means became, not was ...-ed.

So this is better understood as a change-of-state sentence, not a true passive.

Retourzending is a compound noun:

• retour = return
• zending = shipment / dispatch

So retourzending means return shipment or returned parcel/shipment, depending on context.

Dutch often forms nouns by combining words into one single word, so writing it together is normal:

• lever + tijd → levertijd
• retour + zending → retourzending

Both nouns are de-words:

• de retourzending
• de levertijd

For compound nouns, the grammatical gender usually comes from the last part of the compound:

• zending is a de-word → retourzending is also a de-word
• tijd is a de-word → levertijd is also a de-word

This is a useful rule for Dutch compounds.

Because langer is part of the predicate, and in Dutch the non-finite or descriptive part often comes later in the clause.

The basic structure is:

• De levertijd werd langer.

Here:

• de levertijd = subject
• werd = finite verb
• langer = predicate adjective/complement

When another phrase is moved to the front, langer still stays near the end:

• Door de retourzending werd de levertijd langer.

That final position is very normal in Dutch.

Yes. Langer is the comparative of lang.

• lang = long
• langer = longer
• het langst = longest

So:

• De levertijd is lang. = The delivery time is long.
• De levertijd werd langer. = The delivery time became longer.

Dutch often forms the comparative by adding -er, just like English often adds -er.

Yes. A more direct version would be:

• De retourzending verlengde de levertijd.

That means: The return shipment lengthened the delivery time.

The original sentence is a bit less direct and focuses on the result:

• Door de retourzending werd de levertijd langer.

The alternative focuses more on the cause as an active force:

• De retourzending verlengde de levertijd.

Both are natural, but the original may sound slightly more neutral or formal.

Often yes.

For example:

• Vanwege de retourzending werd de levertijd langer.

This also means because of / due to the return shipment.

A rough difference:

• door is very common and natural
• vanwege can sound a bit more formal or written

In many contexts, both are possible.