Jeg åbner appen på min mobil, når jeg skal betale regningen.

Åbner is the present tense form of at åbne (to open).

So:

• at åbne = to open
• jeg åbner = I open / I am opening

In this sentence, Danish uses the present tense to describe something that happens regularly or whenever the situation comes up:

• Jeg åbner appen ... når jeg skal betale regningen.
• I open the app ... when I need to pay the bill.

This is very natural in Danish, just as English can use the present for habits.

Because appen is the definite form: the app.

In Danish, the definite article is often attached to the end of the noun:

• en app = an app
• appen = the app

So Danish usually does not say the app as two separate words. Instead, it adds -en to the noun.

Danish usually puts the at the end of the noun instead of using a separate word.

Examples:

• en app = an app
• appen = the app
• en regning = a bill
• regningen = the bill

This is one of the biggest differences from English.
So:

• appen literally looks like app-the
• regningen literally looks like bill-the

På min mobil means on my phone/mobile and is the normal way to talk about something happening on a device or screen.

So:

• på min mobil = on my phone
• på computeren = on the computer
• på nettet = on the internet

Using i min mobil would usually sound strange here, because you are not physically inside the phone. Danish often uses på where English also uses on for devices and platforms.

Yes. Mobil here means mobile phone / cellphone / phone.

In everyday Danish, mobil is a very common shortened form. You may also hear:

• mobiltelefon = mobile phone
• telefon = phone
• min mobil = my phone

So på min mobil is a very natural everyday expression.

Because når means when in the sense of whenever / at the time that, while hvis means if.

In this sentence, the speaker is talking about a situation that actually occurs:

• når jeg skal betale regningen = when / whenever I need to pay the bill

This suggests a real, expected situation, often a repeated one.

Compare:

• Når jeg skal betale regningen, åbner jeg appen.
  = When/whenever I need to pay the bill, I open the app.

• Hvis jeg skal betale regningen, åbner jeg appen.
  = If I need to pay the bill, I open the app.

The second version sounds more conditional and less like a normal routine.

Skal betale means something like have to pay / need to pay / am going to pay.

So:

• jeg betaler regningen = I pay / am paying the bill
• jeg skal betale regningen = I have to pay / need to pay the bill

In this sentence, skal shows that paying the bill is something the speaker needs to do. It gives the clause a sense of necessity or upcoming action.

That is why når jeg skal betale regningen is very natural for:

• when I need to pay the bill
• when I’m about to pay the bill

Yes. After når, you get a subordinate clause, and Danish subordinate clauses normally keep the subject before the verb:

• når jeg skal betale regningen

Structure:

• når = when
• jeg = I
• skal = must / need to
• betale = pay
• regningen = the bill

That is normal subordinate-clause word order in Danish.

Because the main clause comes first:

• Jeg åbner appen på min mobil, ...
• then the subordinate clause:
• når jeg skal betale regningen

When the sentence starts with the main clause, the main clause keeps normal word order:

• Jeg åbner ...

But if you put the når-clause first, then the main clause would show inversion:

• Når jeg skal betale regningen, åbner jeg appen på min mobil.

Notice:

• Jeg åbner ...
  becomes
• ... åbner jeg ...

That is standard Danish word order.

Yes. Regningen means the bill or the invoice, so it is definite.

• en regning = a bill / an invoice
• regningen = the bill / the invoice

It usually refers to a bill that is already known from the situation. For example:

• your electricity bill
• your phone bill
• a bill you have received and now need to pay

So the sentence is not talking about bills in general, but about the bill that needs to be paid in that situation.

Usually it means a habitual or repeated action:

• I open the app on my phone when I need to pay the bill.

The present tense in Danish often works this way. It can describe:

• habits
• routines
• general truths
• near-future actions, depending on context

Without extra context, this sentence most naturally sounds like a routine.

Yes, you often can.

For example:

• Jeg åbner appen på min telefon ...
• Jeg åbner appen på min mobil ...

Both are natural.
Mobil sounds a bit more specifically like mobile phone, while telefon is broader but often still understood as a mobile phone in everyday speech.

Yes, app is completely normal in modern Danish. It is a common loanword, and people use it all the time.

You will commonly see:

• en app
• appen
• apps or appene depending on context

So in this sentence, appen is perfectly standard everyday Danish.

Yes:

• Når jeg skal betale regningen, åbner jeg appen på min mobil.

This means the same thing. The main difference is emphasis and word order.

With the når-clause first:

• the time/situation is introduced first
• the main clause must use inversion:
• åbner jeg instead of jeg åbner

Both versions are correct and natural.