Learn Hardcore Danish: Planen ændrer sig hurtigt, hvis vejret bliver farligt. - The plan changes quickly if the weather becomes dangerous.

Ændre = “to change (something)”
- Jeg ændrer planen. = I change the plan.
Ændre sig = “to change” (intransitive)
- Planen ændrer sig. = The plan changes.

You normally cannot drop sig here.
Planen ændrer hurtigt sounds wrong in Danish; it feels like something is missing.

Why is it hurtigt and not hurtig?

Hurtig is an adjective (“fast, quick”).
Hurtigt is the adverb form (“quickly, fast”).

In Danish, many adverbs are formed by adding -t to the adjective:

hurtig → hurtigt (quick → quickly)
langsom → langsomt (slow → slowly)

Because hurtigt describes how the plan changes (the manner of the action), the adverb is needed:

Planen er hurtig. = The plan is fast/quick. (adjective, about the plan)
Planen ændrer sig hurtigt. = The plan changes quickly. (adverb, about the changing)

Why is there a comma before hvis in Danish? Is it required?

Yes, in standard written Danish a comma is normally placed before a subordinate clause, and hvis starts a subordinate clause.

So:

Planen ændrer sig hurtigt, hvis vejret bliver farligt.

follows the typical rule:
main clause, subordinate clause.

Many Danes still use this “start comma” (startkomma) as the norm in formal writing. You will also sometimes see the comma omitted in less formal texts, but learning it with a comma is safe and correct.

What is the difference between hvis and når here? Could I say når vejret bliver farligt?

Both hvis and når can translate to “if/when,” but they aren’t interchangeable:

hvis = if (conditional, it might or might not happen)
når = when (something you expect to happen sooner or later)

In your sentence, hvis stresses the condition: if the weather becomes dangerous (maybe it will, maybe it won’t).

If you say:

Planen ændrer sig hurtigt, når vejret bliver farligt.

it suggests that this regularly happens whenever the weather becomes dangerous (more like a general rule or repeated situation).

Why is it vejret and not vejr or vejren?

The base noun is vejr (“weather”), which is neuter:

et vejr (a weather) – rarely used like this, but grammatically correct
vejret (the weather)

Danish neuter nouns form the definite singular with -et, not -en:

et hus → huset (a house → the house)
et vejr → vejret (a weather → the weather)

Vejren would be wrong, because vejr is not a common-gender noun (en vejr does not exist).

Why is it bliver farligt instead of er farligt?

er farligt = is dangerous (state)
bliver farligt = becomes dangerous / gets dangerous (change into a state)

In this sentence we’re talking about the weather turning dangerous, not just being dangerous already. That’s why Danish uses bliver:

Hvis vejret er farligt = If the weather is (already) dangerous
Hvis vejret bliver farligt = If the weather becomes dangerous

Can I switch the order of the two clauses, like in English?

Yes. You can say:

Planen ændrer sig hurtigt, hvis vejret bliver farligt.
Hvis vejret bliver farligt, ændrer planen sig hurtigt.

Both are correct and mean the same thing.

Note: when the hvis-clause comes first, the main clause still starts with the verb in second position:

Hvis vejret bliver farligt, ændrer planen sig hurtigt.
(1st element: the whole hvis-clause, 2nd element: ændrer)

Why is it farligt and not farlig or farlige?

Farlig is the basic adjective “dangerous.”
Danish adjectives change form based on gender/number/definiteness.

Here, farligt matches vejret:

vejr is neuter (et-vejr-word)
neuter singular indefinite adjective form often adds -t

So:

et farligt vejr = a dangerous weather
vejret er farligt = the weather is dangerous

Other forms for comparison:

en farlig plan = a dangerous plan (common gender)
farlige planer = dangerous plans (plural)
den farlige plan / det farlige vejr = the dangerous plan / the dangerous weather

What’s the difference between plan and planen?

plan = “plan” (indefinite) → usually “a plan”
planen = “the plan” (definite)

Danish common-gender nouns (en-words) form the definite singular with -en:

en plan → planen (a plan → the plan)
en bil → bilen (a car → the car)

If you wanted to say a plan changes quickly, you would say:

En plan ændrer sig hurtigt, hvis vejret bliver farligt.

Could I say Planen ændres hurtigt instead of ændrer sig hurtigt?

You can say Planen ændres hurtigt, but there is a nuance:

ændrer sig – active, reflexive, very common and neutral:
- Planen ændrer sig hurtigt. = The plan changes quickly.
ændres – passive form, often used when you think of something being changed (by someone or something), or in more formal style:
- Planen ændres hurtigt. = The plan is quickly changed.

In everyday speech, ændrer sig will usually sound more natural in this kind of sentence.

How is ændrer different from at ændre?

Ændre is the infinitive (“to change”), used with at or modal verbs:

at ændre = to change
Jeg vil ændre planen. = I want to change the plan.

Ændrer is the present tense form:

Planen ændrer sig. = The plan changes / is changing.
Jeg ændrer planen. = I (am) change(ing) the plan.

So in your sentence, ændrer is present tense: it tells you what happens when the weather becomes dangerous.