Min ryg bliver stiv, når jeg sidder længe.

Because ryg is a common gender noun in Danish, so it takes min in the singular.

• en ryg = a back
• min ryg = my back

If the noun were neuter, you would use mit instead:

• et ben = a leg
• mit ben = my leg

So the possessive has to match the grammatical gender of the noun.

Bliver is the present tense of at blive, which often means to become.

So here:

• Min ryg bliver stiv = My back becomes stiff

In natural English, that is often translated as My back gets stiff.

This is a very common use of blive in Danish when something changes state:

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

Because stiv is a predicate adjective here: it comes after bliver and describes the subject min ryg.

In Danish, adjectives usually stay in their base form in this position:

• Ryggen er stiv = The back is stiff
• Min ryg bliver stiv = My back gets stiff

Compare that with an adjective placed directly before a noun, where agreement matters:

• en stiv ryg = a stiff back
• et stift led = a stiff joint

So after er, bliver, and similar verbs, the adjective is often just the basic form.

Når is used for something that happens regularly, generally, or whenever something else happens.

Here the meaning is general:

• My back gets stiff when I sit for a long time

That is a repeated or typical situation, so når is correct.

Da is usually used for a specific event in the past:

• Da jeg sad længe i går, blev min ryg stiv
  = When I sat for a long time yesterday, my back got stiff

So:

• når = when / whenever, in general
• da = when, at a specific time in the past

In standard Danish, the verb is at sidde and the present tense is sidder.

• at sidde = to sit
• jeg sidder = I am sitting / I sit

English speakers sometimes expect something closer to Norwegian or Swedish forms, but in Danish the correct form is sidder.

Længe means for a long time or long in the sense of duration.

So:

• jeg sidder længe = I sit for a long time / I am sitting for a long time

It describes how long the action lasts.

A useful contrast is:

• lang = long, as an adjective
• længe = for a long time, as an adverb

Examples:

• en lang dag = a long day
• jeg ventede længe = I waited a long time

Because this is a subordinate clause introduced by når.

In this clause, Danish uses normal subject-verb order:

• jeg sidder længe = I sit for a long time

So:

• når jeg sidder længe = when I sit for a long time

Nothing unusual happens here yet. But if the subordinate clause comes first, the main clause changes word order:

• Når jeg sidder længe, bliver min ryg stiv

Notice that in the main clause after the comma, bliver comes before min ryg. That is because Danish is a V2 language: the finite verb usually comes second in the main clause.

Yes, but the meaning changes slightly.

• Min ryg bliver stiv = My back gets stiff
• Ryggen bliver stiv = The back gets stiff

If you are talking about your own back, min ryg is the natural choice.

Danish does use the definite form of body parts in some situations, especially when ownership is already obvious, but min ryg is very straightforward and natural here.

Yes, but it changes the meaning.

• Min ryg er stiv = My back is stiff
• Min ryg bliver stiv = My back gets stiff / becomes stiff

So:

• er describes a state
• bliver describes a change into that state

In this sentence, bliver fits well because the stiffness happens as a result of sitting a long time.

Because Danish normally places a comma before a subordinate clause.

So in:

• Min ryg bliver stiv, når jeg sidder længe.

the part beginning with når is a subordinate clause, and Danish punctuation usually marks that with a comma.

This is more regular in Danish than in modern English, where the comma is often omitted in similar sentences.

Yes. A very common alternative is:

• Når jeg sidder længe, bliver min ryg stiv.

This means the same thing, but the emphasis is slightly different because the when-clause comes first.

Notice the word order in the main clause:

• bliver min ryg stiv
• not min ryg bliver stiv

That inversion happens because the sentence starts with something other than the subject. This is a key feature of Danish main clauses.

A rough pronunciation guide is:

• Min ryg bliver stiv, når jeg sidder længe
• approximately: meen ryg blai-er steef, nor yai sih-ther leng-uh

A few important notes:

• ryg has a vowel sound that does not exist in English, somewhat like a rounded i
• bliver is often pronounced more like bliver / bli’er in natural speech
• jeg often sounds more like yai or yai
• sidder has a soft d sound, not a strong English d

If you are learning pronunciation, it is especially helpful to listen to native audio for ryg, bliver, and jeg, since those are not pronounced the way an English speaker might expect from the spelling.