Det här kuvertet behöver ett frimärke innan jag kan skicka brevet.

Because kuvert is an ett-word (a neuter noun), so the demonstrative must match it:

• den här
  • en-words
• det här
  • ett-words

So:

• den här boken = this book
• det här kuvertet = this envelope

The word här stays the same.

This is a very common Swedish pattern called double definiteness.

With den här / det här / de här + noun, the noun is usually also in the definite form:

• det här kuvertet
• den här bilen
• de här husen

So Swedish often says the equivalent of this envelope-the, not just this envelope.

Yes. Detta kuvert is correct.

The difference is mainly style:

• det här kuvertet = more common in everyday speech
• detta kuvert = more formal, more written, sometimes more emphatic

Also notice the grammar difference:

• det här kuvertet = double definiteness
• detta kuvert = no extra definite ending on the noun

Because frimärke is an ett-word.

In Swedish, every noun has grammatical gender, usually either:

• en word
• ett word

So you have to learn the noun together with its article:

• ett frimärke = a stamp
• frimärket = the stamp

Unfortunately, there is no completely reliable rule for guessing whether a noun is en or ett, so learners usually memorize the article with the noun.

Because the sentence refers to:

• a specific envelope: kuvertet
• a specific letter: brevet
• but not a specific stamp: ett frimärke

The idea is this envelope needs a stamp — any suitable stamp, not one already identified.

If you meant a particular stamp already known from context, you could say frimärket instead.

Yes. Swedish uses behöva much like English need, and it can be used with things as well as people.

So sentences like these are normal:

• Bilen behöver bensin. = The car needs petrol/gas.
• Växten behöver vatten. = The plant needs water.
• Kuvertet behöver ett frimärke. = The envelope needs a stamp.

It can sound a little like personification, but it is completely natural.

Because innan introduces a subordinate clause.

In Swedish subordinate clauses, the usual order is:

subject + finite verb

So:

• innan jag kan skicka brevet

not:

• innan kan jag skicka brevet

The second order would look like main-clause word order, and that is not used here.

Because kan is a modal verb, and Swedish modal verbs are followed by the infinitive without att.

So:

• jag kan skicka
• jag vill skicka
• jag måste skicka

not:

• jag kan att skicka

This is similar to English, where we say I can send, not I can to send.

Because the speaker means a specific letter, not just any letter.

In context, it is probably the letter that is going into the envelope, so Swedish uses the definite form:

• brevet = the letter

If you were talking more generally, you could say:

• jag vill skicka ett brev = I want to send a letter

But here it is a particular one, so brevet makes sense.

Yes. You could say:

Innan jag kan skicka brevet behöver det här kuvertet ett frimärke.

That is perfectly correct.

When a phrase or clause comes first in a Swedish main clause, Swedish keeps verb-second word order, so the finite verb behöver comes right after that fronted clause.

So the structure becomes:

• Innan jag kan skicka brevet
  • behöver
    • det här kuvertet
      • ett frimärke

Yes, often you could.

• skicka = send
• posta = mail/post

Because the sentence is about an envelope and a stamp, posta would sound very natural:

• Det här kuvertet behöver ett frimärke innan jag kan posta brevet.

But skicka is also completely correct and a bit broader in meaning. It does not focus specifically on the postal system as strongly as posta does.