På mandag starter min arbejdsdag tidligt, så jeg går i seng før klokken ni.

Because Danish follows the V2 rule in main clauses: the finite verb must come in the second position.

Here, På mandag is placed first, so the verb starter must come next:

• På mandag starter min arbejdsdag tidligt

If you put the subject first, the sentence becomes:

• Min arbejdsdag starter tidligt på mandag

Both are correct, but the word order changes because of V2.

Usually på mandag means this coming Monday / next Monday, depending on the context.

It is a very common way to refer to the upcoming Monday from the speaker’s point of view.

Compare:

• på mandag = this coming Monday / next Monday
• om mandagen = on Mondays, on Monday in general
• i mandags = last Monday

So in this sentence, på mandag is talking about one specific Monday in the future.

Danish often uses the present tense to talk about the future when the time is already clear from context.

Here, På mandag already shows that the action is in the future, so Danish does not need a separate future form.

That is very natural in Danish:

• Jeg rejser i morgen = I’m leaving tomorrow
• Vi ses på fredag = We’ll see each other on Friday

So starter and går are present tense forms, but the meaning is future because of På mandag.

Arbejdsdag is a compound noun, which is extremely common in Danish.

It is made from:

• arbejde = work
• dag = day

Together, arbejdsdag means workday / working day.

Danish usually writes compound nouns as one word, where English often uses two words or a hyphen. So a learner should get used to seeing long nouns written together.

Also notice the -s- in the middle: arbejdsdag. This linking s appears in many compounds, though it is not always predictable at first.

Because arbejdsdag is a common gender noun in Danish, so it takes min.

In Danish:

• en-words take min
• et-words take mit

Since it is:

• en arbejdsdag

you say:

• min arbejdsdag

If it were an et-word, you would use mit instead.

Because tidligt is being used as an adverb, not as an adjective describing a noun.

The adjective is:

• tidlig = early

The adverb form is:

• tidligt = early

Here it describes how the workday starts:

• starter tidligt = starts early

This is very common in Danish. Many adjectives add -t when used adverbially:

• hurtig → hurtigt
• langsom → langsomt
• tidlig → tidligt

Så means so, as in therefore / as a result.

It connects the two clauses:

• På mandag starter min arbejdsdag tidligt
• så jeg går i seng før klokken ni

So the logic is:

• My workday starts early on Monday,
• so I go to bed before nine.

This is different from fordi, which means because.
Compare:

• ..., så jeg går i seng tidligt = ..., so I go to bed early
• Jeg går i seng tidligt, fordi min arbejdsdag starter tidligt = I go to bed early because my workday starts early

Because så here is being used as a coordinating conjunction meaning so, and after it you usually keep normal main-clause word order:

• så jeg går i seng...

That means the subject jeg comes before the verb går.

Learners sometimes confuse this with sentence adverbs like derfor (therefore), which do trigger V2 inversion:

• Derfor går jeg i seng før klokken ni

So:

• ..., så jeg går... = correct
• Derfor går jeg... = also correct, but different structure

Yes, more or less. At gå i seng is the normal Danish expression for to go to bed.

It is an idiomatic phrase:

• gå = go
• i seng = into bed / to bed

So even though the wording may feel slightly different from English, you should learn gå i seng as a set expression.

Examples:

• Jeg går i seng nu = I’m going to bed now
• Børnene går tidligt i seng = The children go to bed early

Klokken ni means nine o’clock.

So:

• før klokken ni = before nine o’clock

In Danish, klokken is commonly used when giving the time:

• Klokken er ni = It is nine o’clock
• Vi mødes klokken otte = We’re meeting at eight o’clock

You can sometimes hear a shorter version in casual speech, but før klokken ni is perfectly natural and clear.

Yes, often you can, and both can mean before nine o’clock.

However, there is a small nuance:

• før klokken ni = before nine
• inden klokken ni = by / before nine, often with a sense of no later than

In many everyday sentences, both work. But inden can sound slightly more like a deadline.

So in this sentence:

• før klokken ni sounds very natural for before nine
• inden klokken ni would also be possible, but may feel a bit more goal-oriented

A more neutral version would start with the subject:

• Min arbejdsdag starter tidligt på mandag, så jeg går i seng før klokken ni.

This version is also completely correct.

The original sentence begins with På mandag to emphasize the time. That is a very common reason to put a time expression first in Danish.

So the difference is mainly one of focus:

• På mandag starter... = emphasis on Monday
• Min arbejdsdag starter... = more neutral, subject-first structure

Yes, arbejdsdag is a real and natural word. It means workday / working day.

That said, in everyday speech, people might also phrase the idea differently depending on context, for example:

• Jeg skal tidligt på arbejde på mandag = I have to go to work early on Monday
• Jeg møder tidligt på mandag = I start work early on Monday

So arbejdsdag is perfectly correct, but not the only natural way to express the idea.