I dag lærer vi at bruge “det tager ... at” og “bruge tid på at”, når vi taler om tid.

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

So when I dag (today) is placed first, the verb lærer has to come next:

• I dag lærer vi ...
• literally: Today learn we ...

If you started with the subject instead, you would say:

• Vi lærer i dag ...

Both are correct, but I dag lærer vi ... puts extra focus on today.

Here, lærer is the present tense of lære, and it means learn:

• vi lærer = we learn / we are learning

It does not mean teacher here, even though lærer can also be a noun meaning teacher in other contexts.

So in this sentence:

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

Here at is the infinitive marker, like English to.

So:

• at bruge = to use

In this sentence:

• lærer vi at bruge ... = we learn to use ...

This at does not mean that.

They are both used to talk about time, but they are used differently.

1. det tager ... at

This means it takes ... to ...

It focuses on how much time something requires.

• Det tager en time at lave maden. = It takes an hour to make the food.

2. bruge tid på at

This means spend time doing ...

It focuses on the person who spends the time.

• Jeg bruger en time på at lave mad. = I spend an hour making food.

So the difference is roughly:

• det tager ... at = the task takes time
• bruge tid på at = someone spends time on the task

The ... stands for a time expression.

For example:

• det tager fem minutter at ...
• det tager to timer at ...
• det tager lang tid at ...
• det tager kort tid at ...

So you can insert:

• a specific amount of time
• or a more general expression like long time / short time

Examples:

• Det tager fem minutter at gå derhen. = It takes five minutes to walk there.
• Det tager lang tid at lære dansk. = It takes a long time to learn Danish.

Because bruge tid på is a fixed Danish pattern.

You can think of it as:

• bruge tid på noget = spend time on something
• bruge tid på at gøre noget = spend time doing something

Examples:

• Jeg bruger tid på lektier. = I spend time on homework.
• Jeg bruger tid på at læse. = I spend time reading.

So the på belongs naturally with bruge tid in this expression.

Because after på, the sentence continues with a verb in the infinitive:

• at bruge
• at læse
• at skrive

So:

• bruge tid på at læse = spend time reading

If a noun comes after på, then you do not use at:

• bruge tid på lektier = spend time on homework

But if a verb comes after på, Danish normally uses at:

• bruge tid på at læse
• bruge tid på at øve sig

It means:

• when we talk about time

Here når is used for a general situation or a repeated kind of situation, not one specific past event.

So this sentence means something like:

• Today we are learning to use these expressions when we talk about time.

This is a general teaching context, so når is the natural choice.

A useful contrast:

• når = when in general, whenever
• da = when for a specific past event

The verb tale om means talk about.

So:

• tale om tid = talk about time

The preposition om is just the normal one used with tale in this meaning.

Examples:

• Vi taler om vejret. = We talk about the weather.
• Vi taler om dansk grammatik. = We talk about Danish grammar.

So om tid is not special by itself; it is just the usual pattern tale om + something.

Yes. That is also correct.

Compare:

• I dag lærer vi at bruge ...
• Vi lærer i dag at bruge ...

Both mean essentially the same thing, but the emphasis is slightly different:

• I dag lærer vi ... emphasizes today
• Vi lærer i dag ... is a more neutral subject-first order

Because the original sentence starts with I dag, the verb has to come before the subject: lærer vi.

Because the sentence is mentioning these expressions as grammar patterns, not using them in a normal sentence.

In other words, the lesson is talking about the expressions themselves:

• det tager ... at
• bruge tid på at

This is like in English when a teacher says:

• We are learning it takes ... to and spend time doing.

So they are highlighted because they are the topic of the lesson.

Yes, a little.

After når, you are in a subordinate clause, and Danish does not use the same V2 pattern there.

So you get normal subordinate-clause order:

• når vi taler om tid
• when we talk about time

That is why it is vi taler, not taler vi.

So the sentence shows both patterns:

• main clause: I dag lærer vi ... → V2 word order
• subordinate clause: når vi taler om tid → subject before verb

This is a very common and important difference in Danish grammar.