Du må vedlegge kvitteringen.

Because after a modal verb in Norwegian, the next verb normally stays in the infinitive without å.

So:

• Du må vedlegge kvitteringen. = You must attach the receipt.
• not Du må å vedlegge ...

This is similar to English:

• You must attach ...
• not You must to attach ...

Other common modal verbs that work the same way:

• kan = can
• skal = shall / will / is supposed to
• vil = want to / will
• bør = should
• må = must / have to

Examples:

• Du kan sende den.
• Jeg skal betale.
• Vi bør vente.

Må means must or have to. It expresses necessity or obligation.

So Du må vedlegge kvitteringen means that attaching the receipt is required, not just optional.

Compare:

• Du må vedlegge kvitteringen. = You must / have to attach the receipt.
• Du bør vedlegge kvitteringen. = You should attach the receipt.
• Du kan vedlegge kvitteringen. = You can attach the receipt.

So må is stronger than bør and kan.

Kvitteringen is the definite form of kvittering.

• en kvittering = a receipt
• kvitteringen = the receipt

In Norwegian, definiteness is often shown by adding an ending to the noun.

For this noun:

• kvittering = receipt
• kvitteringen = the receipt

So the sentence refers to a specific receipt, probably one already understood from the situation.

For example:

• Du må vedlegge en kvittering. = You must attach a receipt.
  (any receipt / one receipt)
• Du må vedlegge kvitteringen. = You must attach the receipt.
  (the specific receipt)

Because Norwegian usually puts the definite article at the end of the noun instead of using a separate word like English the.

So:

• en bil = a car

• bilen = the car

• en kvittering = a receipt
• kvitteringen = the receipt

That -en ending is doing the job of English the.

This is one of the big structural differences between English and Norwegian.

Vedlegge means to attach, to enclose, or to include as an attachment, especially with documents, emails, applications, forms, and similar things.

It is a bit formal and very common in written or administrative contexts.

Examples:

• Vedlegg CV-en. = Attach the CV.
• Du må vedlegge dokumentasjon. = You must attach documentation.

A very common alternative is legge ved, which means the same thing and often sounds a bit more natural in everyday Norwegian.

So these are both possible:

• Du må vedlegge kvitteringen.
• Du må legge ved kvitteringen.

They both mean basically the same thing.

They are very close in meaning.

• vedlegge = one-word verb, often a bit more formal
• legge ved = phrasal-style expression, very common and natural

In many situations, they are interchangeable.

Examples:

• Vennligst vedlegg kvitteringen.
• Vennligst legg ved kvitteringen.

Both mean Please attach the receipt.

A learner should mainly recognize both. In practice, legge ved is extremely useful to know because you will see and hear it a lot.

This is normal Norwegian main clause word order.

Pattern:

subject + finite verb + infinitive/main verb + object

So here:

• Du = subject
• må = finite verb
• vedlegge = infinitive/main verb
• kvitteringen = object

This matches the basic English structure pretty closely:

• You must attach the receipt.

Also, Norwegian main clauses usually follow the V2 rule, meaning the finite verb tends to come in the second position. Since Du is first, må comes second.

Yes, du is the normal word for you when speaking to one person, and it is used in most situations, even many polite or professional ones.

Modern Norwegian usually uses du almost everywhere.

There is an older/more formal De, but it is rare in everyday modern Norwegian.

So in a message from a company, form, website, or office, Du må vedlegge kvitteringen is completely normal.

Yes. Vedlegge can be used for both:

• physically enclosing a receipt with papers
• attaching a receipt to an email, form, or online application

The exact meaning depends on context.

For example:

• In a paper application, it may mean enclose the receipt
• In an email or website form, it may mean upload/attach the receipt

So the Norwegian sentence itself does not force only a physical meaning.

A very natural everyday alternative is:

• Du må legge ved kvitteringen.

This means the same thing as Du må vedlegge kvitteringen.

Other possible variations depending on tone:

• Du må sende med kvitteringen. = You have to send the receipt along.
• Husk å legge ved kvitteringen. = Remember to attach the receipt.

So if you want the most practical everyday phrasing, legge ved is a great version to learn.

It is built from:

• kvittering = receipt
• -en = definite ending for a common-gender singular noun

So:

• en kvittering = a receipt
• kvitteringen = the receipt

This noun is common gender in Norwegian, which is why the indefinite article is en and the definite ending is -en.

The same pattern appears in many other nouns:

• en regning → regningen
• en søknad → søknaden
• en billett → billetten

Yes, but it changes the meaning slightly.

• Du må vedlegge kvitteringen. = You must attach the receipt.
• Du må vedlegge en kvittering. = You must attach a receipt.

The first one points to a specific receipt already known in the situation.
The second one is more general and may simply mean that some receipt is required.

So both are grammatical, but they are not exactly the same.