Learn Danish: Tågen danner sig over søen. - The fog forms over the lake.

This is the medio-passive reflexive in action! In Danish, verbs like danne (to form or create) need a reflexive pronoun like sig when an event happens spontaneously, without a specific person or thing causing it. Literally, it translates to 'The fog forms itself'. If you left out sig, it would sound like the fog is actively creating something else.

Could I use the -s passive here and say Tågen dannes instead?

Yes, but there is a subtle difference in feeling. Using the -s passive (dannes) often sounds more like a general, scientific explanation of how fog is created. Using danner sig focuses on the spontaneous event unfolding naturally right now over the lake.

What if the subject wasn't a third-person thing like tågen? Would I still use sig?

No, the reflexive pronoun must always match the subject. We use sig here because tågen acts as 'it' (third-person singular). If the subject were 'I', you would use mig. For example, in the phrase Jeg danner mig et overblik (I form an overview).

Both tågen and søen end in -en. Are they the same type of noun?