Er du sikker på, at bussen kommer til tiden?

Because this is a yes/no question.

In Danish, a normal statement would be:

Du er sikker på, at bussen kommer til tiden.
= You are sure that the bus is coming on time.

To turn that into a yes/no question, Danish usually puts the finite verb first:

• Du er ... → statement
• Er du ...? → question

This is similar to English Are you ...?

Sikker på means sure about or certain about.

It is a fixed combination:

• at være sikker på noget = to be sure about something
• at være sikker på, at ... = to be sure that ...

So in this sentence:

Er du sikker på, at bussen kommer til tiden?

the på belongs with sikker. It is not random; it is the normal preposition used in this expression.

Examples:

• Jeg er sikker på svaret. = I’m sure about the answer.
• Jeg er sikker på, at han kommer. = I’m sure that he is coming.

At introduces a subordinate clause and usually means that.

So:

• Er du sikker på ...? = Are you sure about ...?
• at bussen kommer til tiden = that the bus is arriving on time

In English, that is often omitted:

• Are you sure the bus is coming on time?

In Danish, at is often kept, especially in standard written language.

The comma marks the beginning of the subordinate clause:

Er du sikker på, at bussen kommer til tiden?

The part after at is a full clause: bussen kommer til tiden.

You may see Danish written both with and without this comma, depending on comma style. So learners should know that:

• Er du sikker på, at bussen kommer til tiden?
• Er du sikker på at bussen kommer til tiden?

can both be seen.

In other words, the comma here is about punctuation convention, not a change in meaning.

Because after at, Danish normally uses subordinate clause word order, where the subject comes before the verb.

So:

• bussen = subject
• kommer = verb

That gives:

at bussen kommer til tiden

English does the same:

that the bus comes / is coming on time

The inversion you see in the main question, Er du ...?, does not continue inside the at-clause.

Because Danish often uses the present tense for a future event, especially when it is planned, scheduled, or expected.

So:

bussen kommer til tiden

literally looks like the bus comes on time, but it naturally means:

the bus is arriving on time / the bus will arrive on time

This is very common with timetables, appointments, and arrangements.

Examples:

• Toget går klokken otte. = The train leaves at eight.
• Hun kommer i morgen. = She is coming tomorrow.

Til tiden means on time.

It is a common expression used for arriving or happening at the expected time.

Example:

• Bussen kommer til tiden. = The bus arrives on time.

A useful comparison is i tide, which often means in time in the sense of early enough.

So:

• til tiden = on time
• i tide = in time

Examples:

• Han kom til tiden. = He arrived on time.
• Han kom i tide til mødet. = He arrived in time for the meeting.

Because bussen is the definite form of en bus.

• en bus = a bus
• bussen = the bus

In Danish, the definite article is usually added to the end of the noun:

• en bil → bilen
• en bus → bussen

So bussen means a specific bus, probably one both speakers know about.

It is completely normal everyday Danish.

Er du sikker på, at bussen kommer til tiden? sounds natural in both speech and writing.

In casual speech, people might pronounce parts more loosely, and in informal writing the comma might be left out, but the structure itself is very standard.

Mostly yes, but not perfectly.

Word by word:

• Er = Are
• du = you
• sikker på = sure about
• at = that
• bussen = the bus
• kommer = comes / is coming
• til tiden = on time

A very close English version is:

Are you sure that the bus is coming on time?

But the most natural English translation might drop that:

Are you sure the bus is coming on time?

So the structure is similar, but natural English and natural Danish do not always match word-for-word.