Hun er pålidelig, fordi hun altid svarer, når jeg skriver, og fordi hun husker vores aftaler.

Because pålidelig is an adjective describing hun after the verb er.

After være in Danish, you normally use an adjective by itself:

• Hun er pålidelig
• Han er træt
• De er glade

You would only use en if a noun followed:

• Hun er en pålidelig person

So Hun er pålidelig is the normal structure.

This is adjective agreement.

Here, hun refers to one person, so the adjective stays in its basic form:

• Hun er pålidelig

Other forms would be used in other contexts:

• Det er pålideligt = neuter singular
• De er pålidelige = plural

So in this sentence, pålidelig is the correct form.

The sentence gives two separate reasons, so repeating fordi makes that structure clear and balanced.

The two reasons are:

• fordi hun altid svarer, når jeg skriver
• og fordi hun husker vores aftaler

You could sometimes leave the second fordi out, but repeating it sounds neat and explicit, especially in writing.

Because fordi introduces a subordinate clause, and subordinate clauses do not use normal Danish main-clause word order.

Compare:

• Main clause: Hun svarer altid
• Subordinate clause: fordi hun altid svarer

In subordinate clauses, adverbs such as altid often come before the finite verb. That is why hun altid svarer is the standard written order.

For the same reason: når also introduces a subordinate clause.

In Danish subordinate clauses, the subject normally comes before the verb:

• når jeg skriver
• fordi hun svarer
• hvis de kommer

English speakers often expect something like main-clause word order, but Danish changes the order after conjunctions such as fordi, når, and hvis.

Yes, but the grammar would change.

• fordi introduces a subordinate clause
• for introduces a main clause and means because/for

So you can say:

• Hun er pålidelig, fordi hun altid svarer ...
• Hun er pålidelig, for hun svarer altid ...

Notice the difference in word order:

• after fordi: hun altid svarer
• after for: hun svarer altid

In standard writing, the sentence you were given uses the more straightforward and common explanatory structure.

Because når fits a repeated or regular situation.

Here it means something like whenever / when:

• når jeg skriver = whenever I write to her / when I message her

Compare:

• når = when, in repeated or general situations
• hvis = if, conditional or uncertain
• da = when, for one specific past event

So:

• Når jeg skriver, svarer hun = whenever I write, she replies
• Hvis jeg skriver, svarer hun måske = if I write, she may reply
• Da jeg skrev i går, svarede hun straks = when I wrote yesterday, she replied immediately

Because Danish often uses the simple present for habits, general truths, and repeated actions.

This sentence describes her usual behavior, not just something happening right now:

• she is reliable
• she always replies
• she remembers agreements

So present tense is natural. English works similarly here, although English speakers sometimes expect a progressive form in some contexts. Danish uses the simple present much more broadly.

It can absolutely mean I message, I text, or I write to her, depending on context.

In modern Danish, skrive is often used for written communication in general:

• text messages
• chat messages
• emails
• other written contact

So når jeg skriver does not have to mean sitting down and writing a letter. In everyday speech, it often means when I message her.

Because svare can be used without an object, meaning reply/respond.

So:

• Hun svarer = she replies
• Hun svarer mig = she answers me
• Hun svarer på min besked = she replies to my message

In this sentence, the important idea is simply that she responds, so no object is needed.

Aftaler can mean several related things, such as:

• appointments
• plans
• arrangements
• things you agreed to do

So hun husker vores aftaler means she remembers the plans or arrangements the two people have made together.

Using the plural sounds natural because people usually have more than one arrangement over time.

They mark clause boundaries.

This sentence contains several clauses:

• Hun er pålidelig
• fordi hun altid svarer
• når jeg skriver
• og fordi hun husker vores aftaler

In Danish, many writers use commas before subordinate clauses such as those introduced by fordi and når. This is an accepted style. You may also see slightly fewer commas in other correct Danish writing, depending on the comma system being used.

So the commas are mainly there to show the structure clearly.