Man bliver hurtigt træt, hvis man går sent i seng.

In this sentence, man is an impersonal pronoun. It means something like:

• one
• you in a general sense
• people

So it does not mean man as in adult male.

In everyday English, this kind of Danish man is often most naturally translated as you:

• Man bliver hurtigt træt = You get tired quickly

Because Danish normally needs an explicit subject in each clause.

So the sentence has:

• main clause: Man bliver hurtigt træt
• subordinate clause: hvis man går sent i seng

You cannot normally leave out the second man. English sometimes feels looser about repeating subjects, but Danish is not.

Because blive means become or get, while være means be.

So:

• man bliver træt = one gets tired / becomes tired
• man er træt = one is tired

This sentence is about the change of state: going to bed late causes you to become tired quickly.

Because hurtigt is the adverb form, meaning quickly.

• hurtig = quick / fast as an adjective
• hurtigt = quickly as an adverb

Here it describes how someone gets tired:

• bliver hurtigt træt = gets tired quickly

A very common pattern in Danish is that an adjective gets -t when used adverbially.

It modifies the whole idea bliver træt.

So the meaning is not that the tiredness itself is quick. It means the process happens quickly:

• someone quickly becomes tired

Word-for-word learners sometimes expect something closer to English word order, but Danish often places adverbs like hurtigt before the adjective or participle they relate to.

Because sent is the form used adverbially here, meaning late.

• sen = late as an adjective
• sent = late in adverbial use

So:

• gå sent i seng = go to bed late

This is similar to hurtigt above: Danish often uses the -t form when a word functions like an adverb.

Gå i seng is a fixed expression meaning go to bed.

Literally, it looks like go in bed, but you should learn it as an idiom:

• gå i seng = go to bed

The preposition i is simply the one Danish uses in this expression. Prepositions often do not match English one-for-one, so it is best to learn the whole phrase together.

Also, gå here does not specifically mean walk. It just means go.

Because hvis introduces a subordinate clause.

After hvis, Danish normally uses ordinary subordinate-clause order:

• hvis + subject + verb + other elements

So:

• hvis man går sent i seng

This is different from main-clause word order, where Danish follows the V2 rule.

A useful comparison:

• Man bliver hurtigt træt, hvis man går sent i seng.
• Hvis man går sent i seng, bliver man hurtigt træt.

In the second version, the main clause begins after the subordinate clause, so you get inversion:

• bliver man not
• man bliver

Because Danish, like English, often uses the present tense for:

• general truths
• habits
• typical results

This sentence is not necessarily talking about right now. It states a general rule:

• if a person goes to bed late, that person gets tired quickly

So the present tense is completely natural here.

Yes, but the meaning changes slightly.

• Man bliver hurtigt træt, hvis man går sent i seng.
  = general statement about people in general

• Du bliver hurtigt træt, hvis du går sent i seng.
  = directed specifically at you

Using man sounds more general and neutral. Using du makes it more personal and direct.

In modern Danish, the comma before a subordinate clause like hvis is often used, and it looks very normal here:

• Man bliver hurtigt træt, hvis man går sent i seng.

You may also see it written without that comma, depending on comma style. So for learners, the important thing is:

• the comma here is very common
• seeing it without a comma is also possible in modern Danish

If you are following a course or textbook, it is best to follow that same comma style consistently.