Jeg skriver en lang besked til min søster, før jeg går i seng.

It can mean either one.

In Danish, the simple present tense often covers both:

• I write
• I am writing

So Jeg skriver en lang besked... can mean either a general present action or something happening right now. In this sentence, the context makes I am writing a long message... feel very natural.

If Danish speakers want to make the ongoing meaning extra clear, they might add something like lige nu (right now) or use a phrase such as jeg er ved at skrive.

Because besked is a common-gender noun in Danish.

Danish nouns are divided into two genders:

• common gender → takes en
• neuter gender → takes et

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

• en besked
• et brev

Here, besked is common gender, so it must be en besked.

Because adjectives in Danish change form depending on the noun.

Here, besked is:

• singular
• common gender
• indefinite

For that combination, the adjective usually stays in its basic form:

• en lang besked

Compare:

• et langt brev → neuter singular indefinite
• den lange besked → definite
• lange beskeder → plural

So lang is correct because besked is a singular common-gender noun with en.

In this sentence, besked most naturally means message.

But besked is a flexible word and can also mean things like:

• notice
• information
• instructions
• word/message from someone

Here, because of skriver en lang besked til min søster, the meaning is clearly something like a long message or a long note.

Because til is the normal preposition for the recipient here.

So:

• skrive til nogen = write to someone
• sende noget til nogen = send something to someone

In your sentence:

• Jeg skriver en lang besked til min søster = I am writing a long message to my sister

So til shows who the message is for.

Because in Danish, a possessive like min already makes the noun definite in meaning.

So after possessives, Danish normally uses:

• no separate article
• no definite ending on the noun

That is why you say:

• min søster
• mit hus
• mine venner

Not:

• min søsteren

Also, min is used because søster is singular common gender.

Compare:

• min søster → common singular
• mit værelse → neuter singular
• mine søstre → plural

Yes, you often can.

Both før and inden can mean before in this kind of sentence:

• før jeg går i seng
• inden jeg går i seng

Both are natural Danish.

A simple way to think of it:

• før is very common and straightforward
• inden is also common, sometimes felt as slightly more formal or time-focused

In this sentence, either one works well.

Because før here is a conjunction introducing a subordinate clause.

After a conjunction like før, you get a clause with normal subject + verb order:

• før jeg går i seng

Here:

• jeg = subject
• går = finite verb

So the structure is:

• før + subject + verb

Før går jeg i seng would not mean before I go to bed here. It would be understood differently and is not the correct structure for this sentence.

Gå i seng is a fixed Danish expression meaning go to bed.

Literally, it looks like go in bed, but you should learn it as a set phrase.

Important difference:

• gå i seng = go to bed, turn in for the night
• være i sengen = be in the bed physically

So:

• før jeg går i seng = before I go to bed
• jeg ligger i sengen = I am lying in the bed

Why no article? Danish often leaves out the article in certain fixed expressions involving places, activities, or states. I seng is one of those.

It depends on which Danish comma style is being used.

You may see both:

• Jeg skriver en lang besked til min søster, før jeg går i seng.
• Jeg skriver en lang besked til min søster før jeg går i seng.

Both can be accepted in modern Danish, because Danish allows different comma conventions. Many learning materials include the comma because it clearly marks the subordinate clause.

So if you see the comma here, it is normal. If you do not see it, that can also be normal Danish.