Hvis jeg skal op tidligt, går jeg i seng før klokken ni.

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

The sentence starts with a subordinate clause: Hvis jeg skal op tidligt = If I have to get up early.

After that clause, the main clause begins, and in Danish the finite verb normally comes before the subject when something else has been placed first. So you get:

• Hvis jeg skal op tidligt, går jeg i seng før klokken ni.

This is the same pattern you see in many Danish sentences:

• I morgen går jeg tidligt i seng.
• Når jeg er træt, drikker jeg kaffe.

If the main clause stood alone, it would be:

• Jeg går i seng før klokken ni.

So the word order changes because the sentence begins with Hvis ...

Skal op is a very common Danish way to say have to get up.

Literally, stå op means get up / stand up, and that is the full verb expression. But after skal, Danish often drops stå and just says op:

• Jeg skal op klokken seks. = I have to get up at six.
• Vi skal tidligt op i morgen. = We have to get up early tomorrow.

So:

• skal op = have to get up
• står op = get up

Examples:

• Jeg står op klokken seks hver dag. = I get up at six every day.
• Jeg skal op klokken seks i morgen. = I have to get up at six tomorrow.

Both are natural, but skal op is especially common when talking about needing to wake/get up at a certain time.

Because gå i seng is a fixed expression meaning go to bed.

In Danish, when you talk about the action of going to bed, you usually say:

• gå i seng

not

• gå i sengen

This works like an idiomatic phrase. It focuses on the activity, not on a specific physical bed.

Compare:

• Jeg går i seng nu. = I’m going to bed now.
• Barnet ligger i sengen. = The child is lying in the bed.

So:

• i seng = to bed
• i sengen = in the bed

Før klokken ni means before nine o’clock.

In Danish, when giving a clock time, it is very common to use klokken:

• klokken ni = nine o’clock
• klokken halv otte = half past seven
• klokken tolv = twelve o’clock

So:

• før klokken ni = before nine
• efter klokken ni = after nine
• klokken ni = at nine o’clock

You may sometimes hear time expressions without klokken in other contexts, but with exact clock times, klokken is very standard.

Because it describes when someone has to get up, not a noun.

In the sentence:

• Hvis jeg skal op tidligt ...

tidligt means early, and it modifies the idea of getting up. That makes it an adverb.

Compare:

• en tidlig morgen = an early morning
  Here tidlig is an adjective describing morgen.

• jeg står tidligt op = I get up early
  Here tidligt is an adverb describing the action.

So the -t form is normal when the word functions adverbially.

Danish often uses the present tense for future meaning, especially when the context already makes the future clear.

In this sentence:

• Hvis jeg skal op tidligt, går jeg i seng før klokken ni.

both verbs are present tense:

• skal
• går

But the meaning is about a general rule or future situation: If I have to get up early, I go to bed before nine.

This is very natural in Danish, just as English can also say:

• If I need to get up early, I go to bed early.

Danish does not always need a special future form.

For conditional if, yes, hvis is the normal word.

Examples:

• Hvis det regner, bliver vi hjemme. = If it rains, we’ll stay home.
• Hvis jeg har tid, kommer jeg. = If I have time, I’ll come.

English learners sometimes confuse hvis with om because both can relate to if in English.

A useful distinction is:

• hvis = if, in conditions
• om = whether / if, in indirect questions

Compare:

• Hvis du kommer, bliver jeg glad. = If you come, I’ll be happy.
• Jeg ved ikke, om du kommer. = I don’t know whether/if you are coming.

So in your sentence, hvis is exactly right because it introduces a condition.

Yes, but the meaning changes slightly.

• Hvis = if
  This gives a condition: on the occasions when that condition is true.

• Når = when
  This sounds more definite or expected, as if it really happens.

Compare:

• Hvis jeg skal op tidligt, går jeg i seng før klokken ni.
  = If I have to get up early, I go to bed before nine.

• Når jeg skal op tidligt, går jeg i seng før klokken ni.
  = When I have to get up early, I go to bed before nine.

Both can be natural, but hvis is more purely conditional, while når suggests a repeated or expected situation.

In skal op expressions, tidligt commonly goes after op:

• Jeg skal op tidligt.

That is the most natural order.

You may also see adverbs in other positions depending on emphasis or sentence structure, but for a learner, the safest pattern is:

• subject + skal + op + time/adverb

Examples:

• Jeg skal op tidligt.
• Vi skal op klokken seks.
• Hun skal tidligt op i morgen.

Notice that Hun skal tidligt op i morgen is also possible. That happens because adverb placement can vary. But skal op tidligt is a very common and easy pattern to learn first.

It can mean either, depending on context.

By itself, the sentence most naturally sounds like a general rule or usual practice:

• Hvis jeg skal op tidligt, går jeg i seng før klokken ni. = Whenever I have to get up early, I go to bed before nine.

But in the right context, it could also refer to a particular upcoming situation.

For example:

• I morgen skal jeg være på arbejde klokken seks. Hvis jeg skal op tidligt, går jeg i seng før klokken ni.

So Danish present tense can cover:

• habits
• general truths
• planned future actions

Context tells you which one is meant.

A careful pronunciation would be roughly:

• Vis yai skal op TID-ligt, gor yai i seng før KLOK-en nai

A few useful points:

• Hvis is often pronounced almost like vis
• jeg often sounds closer to yai
• går has a soft, rounded vowel and may sound a bit like gor, though not exactly like English gor
• før has a front rounded vowel that English does not really have
• ni sounds like nee

The natural rhythm puts stress especially on:

• tidligt
• seng
• ni

If you want to sound natural, focus especially on learning these chunks:

• hvis jeg skal op tidligt
• går jeg i seng
• før klokken ni