Jag hittade ett gammalt frimärke i lådan, men kuvertet var redan stängt.

Because frimärke is an ett-word, also called a neuter noun.

In Swedish:

• en is used with common-gender nouns
• ett is used with neuter nouns

The adjective must agree with the noun, so:

• en gammal bok = an old book
• ett gammalt frimärke = an old stamp

The -t on gammalt shows that the noun is singular, indefinite, and neuter.

Usually, you have to learn the gender together with the noun: ett frimärke, not just frimärke.

Unfortunately, Swedish noun gender is not always predictable from the ending. A dictionary will normally show it for you. So it is best to memorize nouns with their article:

• ett frimärke
• en låda
• ett kuvert

This is just how that adjective changes in the neuter form.

The base form is gammal, but the neuter singular indefinite form is gammalt. This is a normal pattern for some adjectives ending in -al or similar forms.

Compare:

• en gammal bil
• ett gammalt hus

So gammalt is the correct neuter form, not gammaltt or gammal t.

Because Swedish usually marks the by adding a suffix to the noun.

So:

• låda = a box / a drawer
• lådan = the box / the drawer

This is called the definite form. Swedish often uses the ending instead of a separate word like English the.

You normally use den/det with the definite noun when there is also an adjective:

• den stora lådan = the big box / the big drawer

But without an adjective, lådan by itself is enough.

Because the sentence refers to a specific box or drawer: the box/drawer, not just a box/drawer.

• i en låda = in a box/drawer
• i lådan = in the box/drawer

So the definite form tells you that this is a particular, known container in the situation.

Låda can mean either box or drawer, depending on context.

So i lådan could mean:

• in the box
• in the drawer

Swedish uses the same noun for both, and the surrounding context tells you which meaning is intended.

Because kuvert is a neuter noun, so its singular definite form ends in -et.

Compare:

• ett kuvert = an envelope
• kuvertet = the envelope

This is similar to:

• ett hus → huset
• ett frimärke → frimärket

So kuvertet simply means the envelope.

Because kuvertet is neuter singular, and stängt agrees with it.

Here stängt comes from stänga = to close / shut, and it works like a past participle or adjective meaning closed.

Agreement works like this:

• dörren var stängd = the door was closed
• fönstret var stängt = the window was closed
• dörrarna var stängda = the doors were closed

Since kuvertet is an ett-word, stängt is the correct form.

It comes from a verb, but in this sentence it is functioning like an adjective that describes a state.

In kuvertet var redan stängt, the meaning is that the envelope was in a closed state.

So it is best understood as a past participle used adjectivally after var.

Compare:

• Han stängde kuvertet. = He closed the envelope.
  Here stängde is clearly the verb.
• Kuvertet var stängt. = The envelope was closed.
  Here stängt describes the envelope.

Because in Swedish, adverbs like redan often come after the finite verb and before the adjective or participle.

So the pattern here is:

• subject: kuvertet
• finite verb: var
• adverb: redan
• complement: stängt

This is very natural Swedish word order:

• Han är redan klar.
• Dörren var redan öppen.
• Kuvertet var redan stängt.

Because Swedish often uses the preterite for completed events in a story or narration.

• hittade = found
• var = was

This is very normal when describing what happened:

• Jag hittade...
• Kuvertet var...

Swedish does have a present perfect:

• har hittat
• har varit

But in a straightforward past narrative, the simple past is often the most natural choice.

Because the sentence contains two main clauses joined by men = but:

• Jag hittade ett gammalt frimärke i lådan
• men kuvertet var redan stängt

A comma before men is very common and helps show the pause between the two clauses. You may sometimes see shorter sentences without a comma, but this punctuation is completely standard and clear.

Yes, some parts can move, but Swedish still follows the verb-second rule in main clauses.

For example, you could say:

• I lådan hittade jag ett gammalt frimärke, men kuvertet var redan stängt.

Notice what happens:

• I lådan moves to the front
• the verb hittade must still come second
• the subject jag comes after the verb

That verb-second pattern is one of the most important features of Swedish word order.