Hold dig varm, hvis du går ud i kulden.

Because Hold is the imperative form, used for commands or advice.

• holde = to keep
• hold! = keep!

So Hold dig varm means something like Keep yourself warm.

If you said Du holder dig varm, that would mean You keep yourself warm, which is a normal statement, not a command.

Dig is the object form of du and is used because the expression is holde sig varm = to keep oneself warm.

So:

• du = you
• dig = you / yourself
• holde dig varm = keep yourself warm

English often uses yourself for emphasis, but Danish very naturally uses dig in this kind of structure.

Here varm is a predicative adjective, describing the person referred to by dig.

With a person like du, Danish normally uses the basic form:

• Jeg er varm
• Du er varm
• Hold dig varm

Varmt would usually be used with a neuter noun or in some adverb-like uses:

• et varmt bad = a warm bath
• Det er varmt = it is warm

So in this sentence, varm is the correct form because it describes you, not a neuter noun.

Yes, it is a very common and natural expression.

It means:

• holde sig varm = keep oneself warm / stay warm

You can change the pronoun depending on who it refers to:

• Jeg holder mig varm = I keep myself warm
• Hun holder sig varm = she keeps herself warm
• Hold jer varme = keep yourselves warm

So the pattern is very useful to learn.

Hvis means if, while når usually means when.

In this sentence, hvis du går ud i kulden presents a condition:

• if you go out into the cold

That is why hvis fits well.

A rough contrast:

• Hvis du går ud i kulden, hold dig varm = if you go out into the cold, keep warm
• Når du går ud i kulden, så hold dig varm = when you go out into the cold, keep warm

Sometimes both can be possible depending on context, but hvis is the clearer conditional choice here.

Because hvis du går ud i kulden is a subordinate clause, and Danish subordinate clauses keep the subject before the verb in this basic pattern:

• hvis
  • subject + verb

So:

• hvis du går ud i kulden

This is different from a main clause, where Danish often has verb-second word order.

For example:

• Du går ud i kulden = You go out into the cold
• Hvis du går ud i kulden = If you go out into the cold

Literally, it is something like goes out into the cold.

It can be understood in parts:

• går = go / walk
• ud = out / outside
• i kulden = into the cold / in the cold

Together, gå ud often means go outside. Adding i kulden makes it more specific: outside into the cold weather.

So the phrase is very natural Danish for going outdoors when it is cold.

Because gå ud works as a natural verb + directional particle combination meaning go out or go outside.

Then i kulden adds more information about where:

• gå ud = go out
• gå ud i kulden = go out into the cold

This order is normal in Danish. The particle ud stays close to the verb, and then the phrase with i explains the destination or environment.

Kulde means cold as a noun.

• kulde = cold
• kulden = the cold

The ending -en is the definite article for a common-gender noun, attached to the end of the word.

So:

• i kulde would be unnatural here
• i kulden = in the cold / into the cold

Danish usually puts the definite article at the end of the noun instead of using a separate word like English the.

Sometimes yes, but the meaning would not be exactly the same.

• gå udenfor = go outside
• gå ud i kulden = go out into the cold

Ud i kulden focuses specifically on the cold conditions, while udenfor only means outside.

So går ud i kulden is more vivid and more directly connected to the advice Hold dig varm.

Yes. The pronoun and adjective would change.

To one person:

• Hold dig varm = keep yourself warm

To more than one person:

• Hold jer varme = keep yourselves warm

Why?

• jer = you / yourselves, plural
• varme = plural form of varm

So a plural version of the whole sentence would be:

• Hold jer varme, hvis I går ud i kulden.

It is neutral and natural everyday Danish.

Because it uses du and dig, it is the normal modern way to speak to one person. That is not considered rude; it is just standard Danish.

A very formal version with De is possible but uncommon in everyday speech.

In this sentence, går is best understood as go.

Although gå often literally means walk, Danish also uses it very commonly in situations where English would simply say go, especially in everyday expressions like:

• gå ud = go out
• gå hjem = go home

So you should not read går ud i kulden too narrowly as only walk out into the cold. It can simply mean go out into the cold.