Learn Danish: Alting ændrer sig når ilden kommer. - Everything changes when the fire comes.

In Danish, the verb ændrer usually means 'changes' in the sense of changing something else. To say that a situation or thing changes on its own, Danish uses the reflexive pronoun sig (itself) to mark a spontaneous event. Literally, the sentence is saying 'Everything changes itself'.

Could I just say Alting ændrer når ilden kommer without sig?

No, that would sound incomplete to a Danish speaker. Without sig, the verb expects a direct object to be acted upon (like Han ændrer planen - He changes the plan). When the subject undergoes the change naturally without acting on something else, you must include sig.

Why is the word for 'when' når here instead of da?

In Danish, the word for 'when' depends on time. We use når for things happening in the present, the future, or for general truths that happen repeatedly. Because this sentence describes a general truth or present condition ('when the fire comes'), we use når. You only use da for a single, specific event in the past.

Could I use alt instead of alting to say 'everything' here?

Yes, you could safely say Alt ændrer sig. Both alt and alting mean 'everything' and are often interchangeable. Alting sounds slightly more tangible, like 'all things' collectively, but in this context they mean the exact same thing and are both perfectly natural.