Jeg putter brevet i en kuvert, før jeg går til postkassen.

Yes. Putter is the present tense of at putte, and it is a very common everyday verb meaning to put, to place, or to tuck/insert something somewhere.

In this sentence, it fits well because you are putting something into something else.

It is slightly more general and colloquial than some other Danish placement verbs:

• lægge = lay/put something down
• sætte = set something upright
• putte = put/place, especially into something

So Jeg putter brevet i en kuvert sounds very natural.

Because brevet means the letter, while brev means a letter or just letter in a general sense.

Danish usually makes nouns definite by adding the definite ending directly to the noun:

• et brev = a letter
• brevet = the letter

Since brev is a neuter noun, its definite ending is -et.

Because kuvert is a common gender noun in Danish.

Danish nouns have two genders:

• common gender → takes en
• neuter → takes et

So:

• en kuvert = an envelope
• kuverten = the envelope

Unfortunately, noun gender usually has to be learned word by word.

Because i means in or into, and that is the normal preposition when something is being placed inside an envelope.

With verbs like putte, Danish often uses i where English may use in or into:

• putte noget i en pose = put something in/into a bag
• putte brevet i en kuvert = put the letter in/into an envelope

If you wanted to strongly emphasize movement inward, Danish can also use ind i, but here plain i is the most natural choice.

Because før jeg går til postkassen is a subordinate clause.

The comma marks the beginning of that clause:

• Jeg putter brevet i en kuvert = main clause
• før jeg går til postkassen = subordinate clause

One useful thing to know is that in modern Danish, the comma before a subordinate clause is sometimes optional, depending on the comma style being used. So you may also see:

Jeg putter brevet i en kuvert før jeg går til postkassen.

Both can be acceptable.

Because after før, you have a subordinate clause, and subordinate clauses normally keep the order subject + verb:

• før jeg går = before I go

The reversed order går jeg is typical of a main clause when something else comes first:

• Nu går jeg = Now I go / Now I’m leaving

So:

• før jeg går = correct
• før går jeg = not correct here

Because the second part is a new clause, and it needs its own subject.

So Danish says:

• Jeg putter ... før jeg går ...

Just like English says:

• I put ... before I go ...

You normally cannot leave out jeg there in standard Danish.

Because Danish often uses the present tense for future or planned actions when the time relationship is already clear from the sentence.

Here, før already shows the sequence of events, so jeg går naturally means before I go / before I’m going to go.

This is very normal in Danish:

• Jeg kommer i morgen = I’m coming tomorrow
• Vi ses senere = We’ll see each other later

So går does not have to mean only go right now.

Because til means to/toward, and the sentence is talking about going to the mailbox, not going into it.

• gå til postkassen = go to the mailbox
• lægge brevet i postkassen = put the letter in the mailbox

So the sentence first says you put the letter into an envelope, and then later you go to the mailbox.

Because postkassen means the mailbox, which sounds more specific and natural here.

It usually refers to the mailbox that is understood from the situation:

• the usual mailbox near your home
• the mailbox you normally use
• a specific mailbox already known in context

If you said til en postkasse, it would mean to a mailbox, which sounds less specific and less natural in this sentence.

Yes, often it can.

You can say:

• før jeg går til postkassen
• inden jeg går til postkassen

Both can mean before I go to the mailbox.

In everyday Danish, they often overlap. But før is a very direct and common way to introduce a clause like this. Inden can sometimes feel a bit more like before the time when, but in a sentence like this, both are natural.

Because Danish usually writes compound nouns as a single word.

So:

• post
  • kasse = postkasse

Then the definite ending is added to the whole compound:

• postkasse = mailbox
• postkassen = the mailbox

This is very common in Danish, even where English often uses two words:

• togstation = train station
• sommerferie = summer holiday
• hoveddør = front door