Voor het onbeschadigde product werd alles terugbetaald.

Here voor means for in the sense of for that item / in relation to that item.

So Voor het onbeschadigde product means something like:

• for the undamaged product
• with regard to the undamaged product
• for that undamaged item

It does not mean before here.

Because product is a het-word in Dutch.

So:

• het product = the product
• een product = a product

This is something you usually have to learn with the noun.

Because the adjective comes before the noun and is used with a definite article: het.

Base adjective:

• onbeschadigd = undamaged

Before the noun:

• het onbeschadigde product

In Dutch, adjectives before a noun usually take -e. A very important exception is with an indefinite singular neuter noun, for example:

• een onbeschadigd product

But with het or de, you normally get:

• het onbeschadigde product
• de onbeschadigde doos

It is built from:

• beschadigd = damaged
• on- = un- / not

So onbeschadigd literally means not damaged or undamaged.

Dutch often puts a different element first to set the topic or emphasis.

So this sentence starts with the prepositional phrase:

• Voor het onbeschadigde product

This gives special attention to that part, as if the speaker is contrasting it with something else, for example damaged products.

A more neutral order would be:

• Alles werd terugbetaald voor het onbeschadigde product.

Both are grammatical, but the original sentence emphasizes for the undamaged product.

Because Dutch main clauses follow the verb-second rule.

That means the finite verb must be in the second position. If something other than the subject comes first, the verb still has to stay second.

Here the first element is:

• Voor het onbeschadigde product

So the verb must come next:

• Voor het onbeschadigde product werd ...

Then the subject comes after that:

• Voor het onbeschadigde product werd alles terugbetaald.

If the subject came first, then you would get:

• Alles werd terugbetaald voor het onbeschadigde product.

Yes. It is the past passive.

Structure:

• werd = was
• terugbetaald = paid back / refunded

So:

• werd terugbetaald = was refunded / was paid back

An active version would be something like:

• Men betaalde alles terug voor het onbeschadigde product.
• Ze betaalden alles terug voor het onbeschadigde product.

In the passive sentence, the focus is on what was refunded, not on who refunded it.

Because the subject is alles, and alles takes a singular verb in Dutch.

So:

• alles werd = everything was

Even though alles refers to many things or the whole amount, grammatically it behaves as singular here.

In Dutch main clauses with an auxiliary or passive construction, the finite verb goes in second position, and the past participle usually goes near the end.

So the pattern is:

• first element
• finite verb
• subject/other elements
• past participle at the end

Here:

• Voor het onbeschadigde product = first element
• werd = finite verb
• alles = subject
• terugbetaald = past participle

That is why the sentence ends with terugbetaald.

Yes, terugbetalen is a separable verb.

Infinitive:

• terugbetalen = to pay back / refund

In a main clause, it can split:

• Ik betaal het geld terug. = I pay the money back.

Its past participle is:

• terugbetaald

That form makes sense because the base verb is betalen, whose participle is betaald. So you get:

• terug + betaald = terugbetaald

Here alles means everything or the full amount.

In refund contexts, it often has the sense of:

• everything was refunded
• the whole amount was paid back
• a full refund was given

So it does not necessarily mean every object in the world; it means everything relevant in this situation.