Pulsen bliver lavere, når jeg hviler mig efter træningen.

Pulsen is the definite singular form of puls: pulse / the pulse.

In Danish, it is very common to use the definite form for body parts when the owner is already obvious from context. So instead of saying min puls (my pulse), Danish often says pulsen (the pulse).

So:

• pulsen = the pulse
• min puls = my pulse

Both can be possible, but pulsen sounds very natural here.

Here bliver means becomes or gets.

So:

• Pulsen bliver lavere = The pulse gets lower / becomes lower

This is a very common use of blive in Danish:

• Det bliver koldt = It gets cold
• Jeg bliver træt = I get tired

So bliver is not just stays or is here—it shows a change.

Lavere is the comparative form of lav.

• lav = low
• lavere = lower

The sentence describes a change relative to an earlier state: the pulse becomes lower than it was before. That is why Danish uses the comparative form.

Compare:

• Pulsen er lav = The pulse is low
• Pulsen bliver lavere = The pulse gets lower

For things like pulse, temperature, level, pressure, Danish usually uses lav / lavere rather than mindre.

So:

• lav puls = low pulse / low heart rate
• pulsen bliver lavere = the pulse gets lower

Mindre usually means smaller / less in a more general quantity sense. It would sound less natural here.

Når is used for general, repeated, or habitual situations—something that happens whenever that situation occurs.

So this sentence means something like:

• My pulse gets lower when I rest after training
• in the sense of whenever I do that

This is different from da, which is often used for a specific past event:

• Da jeg kom hjem, var han træt = When I came home, he was tired

So here når fits because the sentence expresses a general fact or repeated pattern.

The verb here is the reflexive expression at hvile sig, which means to rest.

So:

• jeg hviler mig = I rest
• literally: I rest myself

In Danish, some verbs are commonly used with a reflexive pronoun, even though English does not use one in the same way.

Reflexive forms:

• jeg hviler mig
• du hviler dig
• han/hun hviler sig
• vi hviler os

This is just the normal idiomatic way to say rest in this kind of sentence.

Usually, for the meaning I am resting, Danish prefers jeg hviler mig.

Hvile without sig can exist, but it often sounds different or more formal/literary, or it can mean something like be resting / lie resting / be supported on, depending on context.

So for everyday Danish, if you want to say I rest after training, jeg hviler mig is the safest and most natural choice.

Efter træningen means after the training / after the workout, referring to a specific training session or the training already understood from context.

• træning = training
• træningen = the training / the workout

Danish often uses the definite form when the situation is specific and known.

Compare:

• efter træningen = after the workout / after the training session
• efter træning = after training, more general

Both can be possible, but efter træningen sounds like a particular workout session.

This part is a subordinate clause introduced by når.

The order is:

• når
  • subject
    • verb
      • other elements

So:

• når jeg hviler mig efter træningen

This is different from main-clause Danish, where the verb is typically in second position.

Main clause:

• Pulsen bliver lavere

Subordinate clause:

• når jeg hviler mig efter træningen

Notice that after når, Danish does not invert the subject and verb.

Yes. A few natural alternatives are possible, depending on style:

• Min puls bliver lavere, når jeg hviler mig efter træningen.
• Pulsen falder, når jeg hviler mig efter træningen.
• Pulsen går ned, når jeg hviler mig efter træning.

Differences:

• min puls makes the ownership more explicit
• falder = falls
• går ned = goes down, a bit more everyday

The original sentence is perfectly natural, but Danish often allows several ways to express the same idea.

Normally it sounds like a general fact or a habitual pattern:

• My pulse gets lower when I rest after training

Because of når, the sentence suggests something that happens whenever this situation occurs.

If you wanted to describe one specific past event, Danish would usually be phrased differently, often with a past-tense structure and possibly da instead of når, depending on the context.