Jeg advarer dig mod kulden om natten.

Because jeg is the subject form, like English I.

In Jeg advarer dig ..., the person doing the action is I, so Danish uses jeg.

• jeg = I
• mig = me

So:

• Jeg advarer dig = I warn you
• not Mig advarer dig

Because dig is the object form, like English you in I see you.

In this sentence, you are receiving the warning, so Danish uses dig.

• du = you as subject
• dig = you as object

Compare:

• Du advarer mig = You warn me
• Jeg advarer dig = I warn you

Advarer is the present tense of advare.

Danish verbs are much simpler than English verbs in this way: the verb usually has just one present-tense form for all persons.

So:

• at advare = to warn
• jeg advarer = I warn
• du advarer = you warn
• han/hun advarer = he/she warns

Unlike English, Danish does not add a special -s for he/she/it.

Because advare nogen mod noget is a very common Danish pattern.

It means something like:

• warn someone against something
• warn someone about a danger

So here:

• Jeg advarer dig mod kulden = I warn you against the cold

This is an idiomatic combination, so it is best to learn it as a chunk:

• advare nogen mod noget

There is also advare om, but that is often used a bit differently, more like giving notice or warning that something is happening. In this sentence, mod fits naturally because kulden is the danger you should be careful about.

Kulden is the definite form: the cold.

• kulde = cold
• kulden = the cold

In Danish, the definite form is often used where English might also say the cold, especially when talking about a specific, understood condition or threat.

So mod kulden suggests:

• the cold as a real, known danger in the situation

You may also see mod kulde in other contexts, especially more general or technical ones, but mod kulden sounds very natural here.

Om natten means at night or during the night.

This expression is used for something that happens generally or habitually in the nighttime.

Examples:

• Det er koldt om natten. = It is cold at night.
• Jeg arbejder om natten. = I work at night.

So in your sentence, om natten tells you when the cold is relevant.

It does not usually mean one specific night. For one specific night, Danish would more often use something else, depending on context, such as i nat for tonight.

Because om natten is the normal Danish expression for at night / during the night in a general sense.

I natten is usually not the standard way to say this. It can appear in special or poetic contexts, but it is not the normal everyday choice here.

A useful contrast is:

• om natten = at night, generally
• i nat = tonight, one specific upcoming or current night

So:

• Det bliver koldt om natten. = It gets cold at night.
• Det bliver koldt i nat. = It will be cold tonight.

Because Danish often places time expressions later in the sentence, especially after the main object or prepositional phrase.

So this order is very natural:

• Jeg advarer dig mod kulden om natten.

A rough structure is:

• Jeg = subject
• advarer = verb
• dig = object
• mod kulden = prepositional phrase
• om natten = time expression

You could move parts around in some contexts for emphasis, but the given order is neutral and natural.

Yes. Jeg advarer is present tense, but like the English present tense, it can describe:

• something happening right now
• a general statement
• something habitual or repeated

So depending on context, it could mean:

• I am warning you
• I warn you
• I warn you regularly/about this kind of thing

Danish present tense often covers both the simple present and some uses of the English present progressive.

It is possible in some contexts, but it changes the feel a bit.

• advare nogen mod noget = warn someone against something, stress the danger
• advare nogen om noget = warn someone about something, often more informational

So in your sentence, mod kulden is especially natural because the cold is presented as a danger you should protect yourself from.

If you want the most idiomatic version for this meaning, advare dig mod kulden is the safer choice.

Yes: both kulden and natten show a very common Danish pattern: the definite article is attached to the end of the noun.

• kulde → kulden = the cold
• nat → natten = the night

This is one of the big differences from English. Instead of putting the before the noun, Danish often adds an ending.

So learners should notice that -en here is doing the job that English the does.