I dag lærer vi, hvordan man bruger komma og punktum, når man skriver længere beskeder.

Because Danish main clauses usually follow the V2 rule: the finite verb comes in the second position.

So:

• I dag = first element
• lærer = second element, so it must come here
• vi = comes after the verb

That is why I dag lærer vi is correct.

Compare:

• Vi lærer i dag ... = normal subject-first order
• I dag lærer vi ... = time expression first, so the subject moves after the verb

This is very common in Danish.

Man is a very common Danish pronoun meaning one, you, or people in general.

In this sentence, it does not mean a male person. It is generic.

So:

• hvordan man bruger komma og punktum = how one uses commas and periods
• In natural English, this is often translated as how you use commas and periods

Danish uses man a lot in general statements, where English often prefers you.

Because this is an embedded question or indirect question, not a direct question.

• Direct question: Hvordan bruger man komma og punktum?
• Embedded question: ... hvordan man bruger komma og punktum

In Danish, direct questions often have verb before subject, but embedded questions usually have subject before verb.

So:

• hvordan bruger man ...? = direct question
• hvordan man bruger ... = indirect question

That difference is very important in Danish word order.

Those commas mark the beginning of subordinate clauses:

• hvordan man bruger komma og punktum
• når man skriver længere beskeder

In Danish, many writers use commas to separate subordinate clauses like this.

A useful extra note: Danish has had two accepted comma systems, so in some styles the comma before a subordinate clause can be omitted. But the version in your sentence is completely normal and easy to understand.

So the commas are there because the sentence contains extra clause units introduced by hvordan and når.

Here når means when in a general or repeated sense.

The idea is:

• when you write longer messages
• meaning whenever you write them, or in that situation in general

Compare:

• når = when, in general / whenever
• da = when, about one specific past event
• hvis = if

So in this sentence:

• når man skriver længere beskeder = when writing longer messages / when one writes longer messages

Using da would sound wrong here, because this is not one specific past occasion.

Because Danish often leaves out the article when talking about things in general, especially in explanations, categories, and school-style language.

So:

• komma og punktum = commas and periods / the comma and the full stop as concepts

If you wanted to talk about one specific punctuation mark, you could use the indefinite article:

• et komma
• et punktum

So the sentence is talking about punctuation generally, not about one single comma or one single period.

Because længere is the comparative form of lang.

• lang = long
• lange beskeder = long messages
• længere beskeder = longer messages

So the sentence specifically means messages that are longer, not just long.

Also, comparative forms like længere do not change the way ordinary adjectives do for gender or number here, so længere beskeder is the natural form.

The basic form is en besked.

Its common forms are:

• en besked = a message
• beskeden = the message
• beskeder = messages
• beskederne = the messages

So længere beskeder simply means longer messages in the plural.

Yes, putting I dag at the beginning gives it prominence.

It highlights today as the frame for the whole sentence:

• I dag lærer vi ... = Today, we are learning ...

Danish often places time expressions first when they set the scene. When that happens, the verb still has to stay in second position, which is why you get:

• I dag lærer vi ...

So it is both natural and slightly emphatic.

Yes. That is completely normal.

In this sentence, the main clause has one subject:

• vi in I dag lærer vi

Then the subordinate clauses have a different, more general subject:

• man in hvordan man bruger ...
• man in når man skriver ...

So the structure is:

• we are learning
• about how people / you / one use punctuation
• when people / you / one write longer messages

The subject can change from clause to clause, just as it can in English.