Wij retourneren het product onder garantie.

Both mean we.

The difference is mainly about stress and style:

• we is the more common unstressed form in everyday speech.
• wij is the fuller form and is often used:
  • for emphasis
  • in more formal writing
  • when the word stands alone or is contrasted with someone else

So this sentence sounds a bit formal/businesslike, which fits the context.

Examples:

• We retourneren het product. = neutral, everyday
• Wij retourneren het product. = slightly more formal or emphatic

Because Dutch main clauses normally follow the verb-second (V2) rule.

In this sentence:

• Wij = first element
• retourneren = finite verb, so it comes second
• het product onder garantie = the rest of the sentence

So the structure is:

Subject + finite verb + rest

If you move another element to the front, the verb still stays in second position:

• Onder garantie retourneren wij het product.

That is very typical Dutch word order.

Retourneren means to return in the sense of sending something back.

It is often used in contexts like:

• shops
• customer service
• orders
• repairs
• warranties

So it sounds fairly formal or commercial.

In more everyday Dutch, people also often use:

• terugsturen = to send back
• terugbrengen = to bring back

So retourneren is correct, but it has a slightly more administrative or business tone.

Because product is a het-word in Dutch.

Dutch singular nouns usually take either:

• de
• het

You have to learn this with each noun.

So:

• het product
• plural: de producten

That plural change is normal: many het-words become de in the plural.

Because onder garantie is a fixed expression.

It works like an idiomatic phrase meaning something like:

• under warranty
• covered by warranty
• as a warranty case

Dutch often leaves out the article in set expressions like this.

So:

• onder garantie = the normal phrase
• onder de garantie would usually sound wrong or would only work in a very special context where you mean a specific guarantee/warranty already mentioned

This is similar to how English says under warranty, not usually under the warranty.

It can match both in a broad sense, but in product/customer-service situations, warranty is usually the best English equivalent.

So in a sentence like this, onder garantie is most naturally understood as:

• under warranty
• as a warranty return
• covered by the warranty

English guarantee is not impossible, but warranty is usually the more idiomatic translation in this kind of context.

It is present tense.

The form retourneren here goes with wij:

• ik retourneer
• jij retourneert
• wij retourneren

In Dutch, the present tense can cover several ideas depending on context, including:

• a general action: we return
• something happening now: we are returning
• sometimes even a planned future action

So without more context, this sentence could mean:

• We return the product under warranty
• We are returning the product under warranty

Dutch does not need a separate form like English are returning.

In practice, it describes the situation of the return: the product is being returned as a warranty matter.

So the idea is not that the product is physically under something. Instead, onder is part of the expression onder garantie.

You can think of it as:

• the product is covered by warranty
• the return is being made on warranty grounds

So it connects to the whole event, not just as a literal description of product.

Yes, it is natural, especially in a formal, written, customer-service, or business context.

Because wij and retourneren are both a bit formal, the sentence sounds like something you might see in:

• an email
• a return form
• a customer-service message
• a company note

More casual alternatives might be:

• We sturen het product onder garantie terug.
• We brengen het product onder garantie terug.
  
  • better if you physically bring it back
• We sturen het product terug vanwege de garantie.

So the original sentence is correct, but it has a more official tone than everyday speech.