Siden hun fik børn, har hendes aftenrutine ændret sig meget.

Yes. In this sentence, siden means since and introduces a time clause:

Siden hun fik børn = Since she had children / Since she became a mother

A useful thing to know is that siden can also be used in other ways in Danish, for example with the meaning ago in expressions like for to år siden = two years ago. But in your sentence, it clearly means since.

Both forms are possible in Danish in the right context, but they give slightly different perspectives.

• hun fik børn = she had children / she got children
  This refers to the point in the past when her life changed.
• hun har fået børn = she has had children / she has become a parent
  This focuses more on the present relevance of that event.

In your sentence, Siden hun fik børn is very natural because it points to the time when things started to change. It works like a reference point: from the time she had children onward.

So Danish often uses the simple past after siden when referring to the starting point in the past.

Because the sentence describes a change that began in the past and is still relevant now.

• har ændret sig = has changed
• This is the Danish present perfect.

The structure is:

Siden hun fik børn, har hendes aftenrutine ændret sig meget.

This is similar to English: Since she had children, her evening routine has changed a lot.

So the timeline is:

• past event: hun fik børn
• ongoing result up to now: har ... ændret sig meget

That is why the present perfect is used in the main clause.

Ændre sig means to change.

In Danish, many verbs can be used with a reflexive pronoun like sig, and that changes the meaning or makes it more natural. Here:

• ændre noget = to change something
• ændre sig = to change / to change itself

So:

• Hun ændrer rutinen = She changes the routine
• Rutinen ændrer sig = The routine changes

In your sentence, hendes aftenrutine ændret sig means that her evening routine itself has changed. The sig is necessary here.

Good question. Danish distinguishes between:

• hendes = her
• sin/sit/sine = his/her/its own

You use sin/sit/sine when the possessor is the subject of the same clause.

Examples:

• Hun vasker sin bil = She washes her own car
• Hun vasker hendes bil = She washes her car (someone else’s car, feminine owner)

In your sentence, the subject of the main clause is hendes aftenrutine, not hun. Since hun is inside the subordinate clause (Siden hun fik børn), you do not use sin here.

So hendes aftenrutine is correct.

Because Danish uses verb-second word order in main clauses.

When the sentence begins with something other than the subject, the finite verb comes next.

Here the sentence starts with the subordinate clause:

Siden hun fik børn, ...

After that, the main clause begins, and the finite verb must come in second position:

har hendes aftenrutine ændret sig meget

So the order is:

1. fronted element: Siden hun fik børn
2. finite verb: har
3. subject: hendes aftenrutine

This is very typical Danish word order.

Compare:

• Hendes aftenrutine har ændret sig meget.
• Siden hun fik børn, har hendes aftenrutine ændret sig meget.

Same idea, different word order because of what comes first.

Børn is plural. The singular is barn.

• et barn = a child
• børn = children

So:

• hun fik et barn = she had a child
• hun fik børn = she had children

In your sentence, børn means she became a parent to more than one child, or more generally that she entered the stage of life of having children.

It is a compound noun, which is very common in Danish.

• aften = evening
• rutine = routine
• aftenrutine = evening routine

Danish often joins nouns together into one word, just like German does. English sometimes writes these as two words, a hyphenated form, or one word depending on the expression, but Danish usually prefers one combined word.

Other examples:

• morgenmad = breakfast literally morning food
• arbejdsdag = workday
• barnevogn = baby carriage / stroller

Meget means a lot or very much here.

In the sentence:

har hendes aftenrutine ændret sig meget

it modifies the whole idea of changing, so it means:

has changed a lot

It comes naturally at the end of the clause in this sentence.

Compare:

• Det har ændret sig meget. = It has changed a lot.
• Hun arbejder meget. = She works a lot.

So here meget is functioning adverbially.

Sometimes Danish can use er ændret, but it is not the same construction.

• har ændret sig = has changed
• er ændret = is changed / has been changed

The second version often sounds more passive or result-focused, and it may suggest that something was changed, possibly by an outside force.

In your sentence, har ændret sig is the natural choice because it describes how her routine has developed over time.

So:

• Hendes aftenrutine har ændret sig meget = natural, everyday Danish
• Hendes aftenrutine er ændret meget = less natural here

Yes, literally fik børn is got children, because få means get. But in natural English, we usually say:

• had children
• became a parent
• started having children

So the Danish verb få is often broader than English get. In many contexts it is best translated differently depending on what sounds natural in English.

Examples:

• Jeg fik en gave. = I got a present.
• Hun fik børn. = She had children / She became a mother

So yes, got children is literal, but not usually the best final translation.

You could say:

Hendes aftenrutine har ændret sig meget, siden hun fik børn.

This means the same thing.

Notice what changes:

• When the sentence starts with the subject, you get normal main-clause order:
  Hendes aftenrutine har ...
• When the sentence starts with the siden clause, the finite verb comes before the subject in the main clause:
  Siden hun fik børn, har hendes aftenrutine ...

So both are correct; the difference is mainly in emphasis and sentence flow.