Sæt et mellemrum efter punktummet, og lad være med at glemme kommaet i den lange sætning.

They are both imperatives, so the sentence is giving instructions.

• sætte → sæt = put
• lade → lad = let

So the sentence is made of two commands:

• Sæt et mellemrum efter punktummet
• lad være med at glemme kommaet i den lange sætning

In Danish, just like in English, commands usually leave out the subject.

• English: Put a space after the period
• Danish: Sæt et mellemrum efter punktummet

The subject is understood as you.

Lad være med at + infinitive is a very common Danish expression meaning:

• don’t ...
• stop ...
• refrain from ...

So:

• lad være med at glemme = don’t forget

You should learn this as a fixed expression, because its literal parts do not match English very neatly.

Both are possible.

• Glem ikke kommaet = Don’t forget the comma
• Lad være med at glemme kommaet = also Don’t forget the comma

The version with lad være med at often sounds a bit more conversational, more idiomatic, or slightly softer/longer than the very direct glem ikke.

So the sentence is perfectly natural, even though it is not the shortest way to say it.

Because after at, Danish normally uses the infinitive.

• at glemme = to forget

So in:

• lad være med at glemme

the verb must stay in the infinitive form glemme.

Because mellemrum is a neuter noun in Danish, so it takes et in the indefinite singular.

• et mellemrum = a space

This word is often used for a typing or punctuation space, not just a physical gap.

Because Danish usually makes nouns definite by adding the definite ending directly to the noun, instead of using a separate word like the.

• punktum = period / full stop
• punktummet = the period / the full stop
• komma = comma
• kommaet = the comma

So:

• efter punktummet = after the period
• glemme kommaet = forget the comma

They are just different noun forms.

• komma becomes kommaet
• punktum becomes punktummet

To an English speaker, punktummet may look strange, but it is the normal definite singular form of punktum. It is best learned as a vocabulary pattern:

• punktum → punktummet

Because og is joining two full clauses, and each clause has its own verb in the imperative:

1. Sæt et mellemrum efter punktummet
2. lad være med at glemme kommaet i den lange sætning

In Danish, it is common to put a comma between coordinated clauses like this. So the comma is there because the sentence contains two separate instructions joined by og.

Because when Danish has a definite noun phrase with an adjective, the pattern is:

• den/det/de + adjective with -e + noun

So:

• en lang sætning = a long sentence
• den lange sætning = the long sentence

Here:

• sætning is a common-gender noun, so it takes den
• the adjective becomes lange

That is why the sentence says i den lange sætning.