I dag lærer vi at bruge “så længe”, når noget gælder i hele den tid, en situation varer.

Because Danish main clauses follow the verb-second rule. The finite verb usually comes in the second position.

So if you begin with I dag at the front, the verb lærer must come next:

• I dag lærer vi ...

If you begin with the subject, you get:

• Vi lærer i dag ...

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

After lære when it means learn to do something, Danish normally uses at + infinitive:

• lære at bruge = learn to use
• lære at læse = learn to read

So at bruge is the infinitive phrase connected to lærer. This works very much like English learn to use.

Because the sentence is talking about the expression itself, not using it normally inside the sentence.

In other words, så længe is being mentioned as a language item the learner is studying. That is why it is set apart typographically.

Here så længe is a fixed expression used about duration: something is true or applies for the whole time that a situation lasts.

It often introduces or connects to a clause about how long something continues:

• Du må blive her, så længe du vil.
• Så længe det regner, bliver vi inde.

So learners should think of it as a chunk, not just as two separate words they translate one by one.

Not exactly.

• mens means while, mainly showing that two things happen at the same time.
• så længe means as long as / for the whole time that, with a stronger idea of duration or continuing validity.

Compare:

• Mens jeg laver mad, hører jeg musik.
  = Two actions happen at the same time.

• Du kan blive her, så længe du vil.
  = The situation continues for that whole period.

So they can overlap sometimes, but they are not interchangeable in every sentence.

Because når is commonly used for general or repeated situations: when/whenever something happens.

Here the sentence is giving a general rule about usage:

• når noget gælder i hele den tid ...

It is not describing one specific past event. That is why når fits better than da, which is typically used for a specific event in the past.

Here gælde means something like:

• to apply
• to be valid
• to hold true

So når noget gælder means when something applies or when something is valid.

This is a very common Danish verb in explanations, rules, and conditions:

• Reglen gælder for alle. = The rule applies to everyone.
• Det gælder kun om sommeren. = It only applies in summer.

Because noget is grammatically singular, so the verb is in the singular present form gælder.

Danish present tense does not change for different persons the way English sometimes does, but the form used here is simply the normal present tense:

• noget gælder
• reglen gælder
• det gælder

So gælder is just the standard present tense form of gælde.

This phrase means throughout the whole time or during the entire period.

It breaks down like this:

• i = in / during
• hele = whole, entire
• den tid = that time / the period

Together, i hele den tid gives the idea of something being true for the full duration of a situation.

Because Danish often allows a time noun to be followed directly by a clause.

So instead of needing an extra linking word, Danish can say things like:

• den dag jeg kom
• den tid han boede der
• hele den tid en situation varer

This is normal Danish. English often wants an extra word such as that or when, but Danish often leaves it out.

The comma marks the beginning of a subordinate clause in the written sentence.

Inside that clause, Danish uses normal subject + verb order:

• en situation varer

not

• varer en situation

That is because the verb-second rule belongs to main clauses. In subordinate clauses, Danish usually keeps the subject before the verb.