Learn Hardcore Norwegian: Jeg strekker ryggen hver morgen fordi den ellers blir stiv. - I stretch my back every morning because otherwise it gets stiff.

Verb + body part pattern
In Norwegian it is very common to use verb + the definite form of a body part (without a possessive) when it is clear that the body part belongs to the subject:
- Jeg vasker hendene. = I wash my hands.
- Hun pusser tennene. = She brushes her teeth.
So:
- Jeg strekker ryggen. literally: I stretch the back → naturally understood as I stretch my back.
Why not jeg strekker meg?
Jeg strekker meg means I stretch (myself) in a more general sense, like stretching your whole body, waking up, yawning and stretching, etc.
Jeg strekker ryggen focuses specifically on the back.
Why not ryggen min?
You can say:
- Jeg strekker ryggen min.
  but in this context it sounds less natural. Norwegians usually avoid possessives with body parts when the owner is obvious. Ryggen alone is more idiomatic here.

Why is it ryggen and not just rygg?

Rygg is the indefinite form (a back), and ryggen is the definite form (the back).

With body parts that belong to the subject, Norwegian normally uses the definite form without a possessive:

Jeg vasker håret. = I wash my hair.
Han brakk armen. = He broke his arm.
Jeg strekker ryggen. = I stretch my back.

So ryggen is definite because we are talking about a specific, known back (the speaker’s back), not just a back in general.

Why is it hver morgen and not hver morgenen?

After hver (each / every), the noun stays in the indefinite singular:

hver dag = every day
hver uke = every week
hver måned = every month
hver morgen = every morning

You never use the definite form after hver, so:

hver morgenen is incorrect.

Can hver morgen be placed somewhere else in the sentence?

Yes. Norwegian word order is flexible for time expressions like hver morgen, but there are preferences:

The version you have:
- Jeg strekker ryggen hver morgen fordi den ellers blir stiv.
Very common alternative, emphasizing the time:
- Hver morgen strekker jeg ryggen fordi den ellers blir stiv.

Both are correct.
What you normally do not say is:

Jeg hver morgen strekker ryggen … (very odd).

So the typical options:

Put hver morgen at the beginning of the sentence, or
Put it after the verb phrase / object, as in the original sentence.

Why is it fordi den ellers blir stiv and not fordi jeg ellers blir stiv?

The pronoun den refers to ryggen (the back), not to jeg (I).

rygg / ryggen is a thing, not a person.
In Bokmål, for things, you normally use den (for masculine/feminine nouns) and det (for neuter nouns), regardless of grammatical gender, when you refer back to them with a pronoun.

So:

ryggen → den
boken → den
bilen → den
huset → det

Therefore:

… fordi den ellers blir stiv. = … because it (the back) otherwise gets stiff.

If you said:

… fordi jeg ellers blir stiv.
that would mean: because otherwise I get stiff (as a person), which is a different meaning.

Why is it den and not denn or han?

Spelling:
The correct pronoun is den. There is no form denn in Norwegian.
Why not han?
- han = he (for male persons)
- hun = she (for female persons)
- den / det = it (for things and animals, in many contexts)
Rygg / ryggen is not a person, so you use den, not han.

What exactly does ellers mean here, and where can it go in the sentence?

Ellers here means otherwise or if not.

… fordi den ellers blir stiv. = … because otherwise it gets stiff.

Placement:

The most natural is exactly as in the sentence:
- den ellers blir stiv
You could also say:
- fordi den blir stiv ellers
  This is possible, but it sounds a bit less smooth; ellers usually comes before the main verb or right after the subject in this kind of clause.

So the original placement den ellers blir stiv is the most idiomatic.

Why is it blir stiv and not er stiv?

bli = to become / to get
være = to be

So:

den blir stiv = it becomes stiff / it gets stiff
den er stiv = it is stiff

In this context you are talking about a change of state that happens if you do not stretch:

If I don’t stretch, my back gets stiff (it becomes stiff). So blir stiv is the natural choice.

If you said:

fordi den ellers er stiv
it would sound more like: because otherwise it just (always) is stiff, describing a permanent state, not a state it develops into.

Does jeg strekker here mean I am stretching right now or that I stretch regularly?

In Norwegian, the simple present can express:

Habitual actions (things you do regularly)
- Jeg trener hver dag. = I work out every day.
- Jeg strekker ryggen hver morgen. = I stretch my back every morning.
Present actions (things happening right now), usually with some time expression or context:
- Jeg sitter og leser. = I am reading.

In this sentence, because of hver morgen, the present tense clearly has a habitual meaning:
I (normally) stretch my back every morning.

Could you also say jeg strekker ut ryggen, and is there a difference?

Yes, you can say:

Jeg strekker ut ryggen hver morgen …

Strekke ryggen vs strekke ut ryggen:

strekke ryggen – perfectly normal, means to stretch the back.
strekke ut ryggen – adds a nuance of stretching (it) out / fully extending it. It can sound a bit more physical or intensive, as if you are really extending the back.

In everyday speech, strekke ryggen is completely natural and probably more common in a simple statement like this.

Could you use tøyer instead of strekker?

Yes, you can:

Jeg tøyer ryggen hver morgen fordi den ellers blir stiv.

Strekke and tøye overlap in meaning, but:

strekke (seg / ryggen / ut) – to stretch, to extend.
tøye (ut) – to stretch as in stretching exercises, flexibility training.

For ordinary everyday language about light stretching, both are acceptable.
If you talk more about exercise / physiotherapy / mobility work, tøye is very common:

Jeg tøyer ut etter trening. = I stretch out after working out.