Det var først da læreren satte et ekstra komma ind, at jeg forstod meningen med hele afsnittet.

Because Danish is using a very common focus structure here:

Det var først da X, at Y
= It was only when X that Y

So det is not referring to a specific thing here. It works more like a formal or dummy subject, helping build the sentence structure.

Compare:

• Da læreren satte et ekstra komma ind, forstod jeg meningen.
  = When the teacher inserted an extra comma, I understood the meaning.

• Det var først da læreren satte et ekstra komma ind, at jeg forstod meningen.
  = It was only when the teacher inserted an extra comma that I understood the meaning.

The second version puts strong emphasis on the timing.

On its own, først often does mean first. But in this construction, it usually means only then, not until, or only when.

So here:

Det var først da ... at ...

means something like:

• It was only when ... that ...
• It wasn’t until ... that ...

So først is not about being “first in a sequence” here. It is about a delay: the understanding happened only at that point.

Because da is used for a specific event in the past, while når is usually used for repeated events, general truths, or the present/future.

Here the sentence refers to one particular past moment:

da læreren satte et ekstra komma ind
= when the teacher inserted an extra comma

That is why da is the natural choice.

Very roughly:

• da = when, at that time in the past
• når = when/whenever, in general or in the future

Examples:

• Da han kom, gik vi.
  = When he came, we left.

• Når han kommer, går vi.
  = When he comes, we’ll leave.

In this pattern, Danish often uses at to introduce the clause that tells what happened:

Det var først da X, at Y

So the at is part of the overall sentence pattern. It is similar to English:

It was only when X that Y

The at helps signal the second clause clearly. It is very common and natural in this structure.

Because both parts are subordinate clauses:

• da læreren satte et ekstra komma ind
• at jeg forstod meningen med hele afsnittet

In Danish subordinate clauses, the subject normally comes before the finite verb:

• læreren satte
• jeg forstod

That is different from main clauses, where Danish usually has verb-second word order.

Compare:

• Main clause: Jeg forstod meningen.
• Subordinate clause: ..., at jeg forstod meningen.

So the word order here is exactly what you would expect in subordinate clauses.

Yes. Sætte ... ind is a verb + particle combination, and here it means to insert.

So:

• sætte et komma ind = to insert a comma

The object can go between the verb and the particle:

• satte et ekstra komma ind

This is very common in Danish.

There is also a one-word verb:

• indsætte = to insert

So you could also say:

• læreren indsatte et ekstra komma

That is a bit more formal or written in tone. Satte ... ind sounds very natural in everyday Danish.

Because komma is a neuter noun in Danish, so it takes et:

• et komma

So:

• et ekstra komma = an extra comma

Also, ekstra does not change form here. It stays ekstra regardless of gender or number in this kind of use.

Both are possible in principle, but meningen med hele afsnittet is the more natural everyday phrasing here.

It literally means:

• the meaning of the whole paragraph/section
• more idiomatically: the meaning of the entire passage

Danish often uses med in expressions like this:

• meningen med livet = the meaning of life
• meningen med teksten = the meaning of the text

So meningen med hele afsnittet is very normal Danish.

Because hele before a definite noun normally works like this:

• hele afsnittet = the whole paragraph/section
• hele bogen = the whole book
• hele dagen = the whole day

You do not normally add det in front in this kind of phrase.

So:

• hele afsnittet = correct
• det hele afsnit = not correct here

This is just a standard Danish pattern you will see very often.

Because the sentence tells a sequence of past events, and Danish often uses the simple past for both when the order is already clear from context.

So:

• læreren satte et ekstra komma ind
• jeg forstod meningen

The structure itself makes the timing clear: first the teacher inserted the comma, and then understanding happened.

If you wanted to emphasize that the inserting happened earlier, Danish could also use a past perfect form in some contexts, but it is not necessary here. The simple past sounds natural and idiomatic.

Afsnit can mean paragraph, section, or sometimes passage, depending on context.

So hele afsnittet could be understood as:

• the whole paragraph
• the whole section
• the entire passage

The exact English choice depends on what kind of text is being discussed. In this sentence, the important point is that the speaker suddenly understood the meaning of the entire piece of text being referred to.