Learn Hardcore Norwegian: Kroppen hennes blir mer fleksibel jo mer hun strekker seg hver dag. - Her body becomes more flexible the more she stretches every day.

After the noun (more neutral, very common in speech):
- kroppen hennes = her body
Before the noun (often more emphatic or contrastive):
- hennes kropp = her body (as opposed to someone else’s body, or emphasizing her)

So:

Kroppen hennes is the most natural, neutral way to say her body here.
Hennes kropp would be understood, but can sound a bit more marked/emphatic.

You cannot use sin here:

Sin/sitt/sine refers back to the subject of the clause.
In the main clause, the subject is kroppen, not she.
Kroppen sin would literally mean its own body, which makes no sense.

If you want to use sin, you’d have to change the structure, for example:

Hun får en mer fleksibel kropp jo mer hun strekker seg hver dag. (She gets a more flexible body the more she stretches every day.)

What does “blir” mean here, and why not “er”?

Blir is the verb “to become” (or “gets” in the sense of “becomes”).

Kroppen hennes blir mer fleksibel …
= Her body becomes / is getting more flexible …

Using er (is) would describe a state, not a change:

Kroppen hennes er mer fleksibel.
= Her body is more flexible (no idea why or how; just a fact)

Because the sentence talks about a gradual change over time (as she stretches every day), blir is the natural choice.

How does the “jo mer … jo mer …” structure work?

Jo … jo … in Norwegian is like “the … the …” in English:

Jo mer hun strekker seg, jo mer fleksibel blir kroppen hennes.
= The more she stretches, the more flexible her body becomes.

In your sentence, the order is just flipped:

Kroppen hennes blir mer fleksibel jo mer hun strekker seg hver dag. Literally: Her body becomes more flexible the more she stretches every day.

Pattern:

jo + komparativ (mer / -ere) + S + V, jo + komparativ + S + V
- Jo mer du øver, jo flinkere blir du.
  The more you practice, the better you become.

You can use mer with adjectives or verbs:

jo mer fleksibel kroppen blir (adjective)
jo mer hun strekker seg (verb phrase: the more she stretches)

Why is it “strekker seg” and not just “strekker”?

Å strekke seg is a reflexive verb here and means to stretch (oneself), like stretching your own body.

Hun strekker seg. = She stretches (herself).
Hun strekker armen. = She stretches the arm. (object armen)

Without seg, strekke needs a direct object:

strekke noe = stretch something (a leg, an arm, a rope, etc.)

In your sentence, she is stretching her own body, so Norwegian uses the reflexive form:

hun strekker seg = she stretches (her body)

Could you leave out “seg” and say “jo mer hun strekker hver dag”?

No, that would be wrong or at least incomplete.

Hun strekker by itself usually sounds like something is missing (she stretches … what?).
You either need:
- a reflexive pronoun: Hun strekker seg.
- or an object: Hun strekker beina / armene.

So the correct form in this meaning is hun strekker seg.

Why is “mer” before “fleksibel”, and what about the second “mer” before “hun strekker seg”?

Norwegian comparatives with mer work like English “more + adjective”:

fleksibel → mer fleksibel = flexible → more flexible
interessant → mer interessant

So:

blir mer fleksibel = becomes more flexible

The second mer is used with a verb phrase, and that’s also normal:

jo mer hun strekker seg
Literally: the more she stretches herself

Here mer has the sense of to a greater extent / more often / more intensely.
So both uses of mer are standard:

mer + adjective (mer fleksibel)
mer + verb phrase (mer hun strekker seg)

Why is “fleksibel” not inflected to match kroppen? Shouldn’t it agree in gender/number?

Predicate adjectives in Norwegian do not change form for gender in the singular. They only change for plural (and sometimes neuter in attributive position).

Here, fleksibel is a predicate after the verb blir:

Kroppen hennes blir mer fleksibel.

Pattern:

Subjekt + er/blir + (mer) + adjektiv
- Huset er stort.
- Jenta er smart.
- Kroppen er fleksibel.

No extra ending in the singular; it stays fleksibel.

In the plural, you’d add -e:

Kroppene deres blir mer fleksible.
(Their bodies become more flexible.)

Could the word order be “Jo mer hun strekker seg hver dag, blir kroppen hennes mer fleksibel”?

Yes, that is perfectly correct and actually quite natural:

Jo mer hun strekker seg hver dag, blir kroppen hennes mer fleksibel.

This just puts the “jo mer …”-clause first. The meaning is the same.

Word order rules still apply:

The “jo mer …” clause counts as one big element in front.
Then in the main clause you get inverted word order (V2 rule):
- … blir kroppen hennes mer fleksibel. (verb blir before subject kroppen hennes)

Why is it “hver dag” and not something like “daglig” or “hver eneste dag”?

All are possible, but they differ slightly in style/nuance:

hver dag = every day (neutral, very common)
- Hun trener hver dag.
hver eneste dag = every single day (stronger emphasis, a bit more dramatic)
- Hun trener hver eneste dag.
daglig = daily (slightly more formal/written style)
- Hun trener daglig.

In your sentence, hver dag matches the everyday, conversational tone best:

Kroppen hennes blir mer fleksibel jo mer hun strekker seg hver dag.

Could you drop “hennes” and just say “Kroppen blir mer fleksibel jo mer hun strekker seg hver dag”?

Yes, that’s possible and can be natural if the context is clear that you are talking about her body.

Kroppen blir mer fleksibel jo mer hun strekker seg hver dag.

This sounds like:

The body becomes more flexible the more she stretches every day
(understood as her body, from context)

If you need to be clear in isolation (no context), kroppen hennes is safer:

Kroppen hennes blir mer fleksibel … = explicitly her body.