Learn Dutch: De hond is dan weer rustig, dan weer wild. - The dog is sometimes calm, sometimes wild.

While soms ... soms simply means "sometimes ... sometimes", dan weer ... dan weer (literally "then again ... then again") emphasizes a continuous, often unpredictable fluctuation between two states. It gives the exact feeling of "one minute he is calm, the next he is wild."

What happens to the word order if I start the sentence with dan weer?

Because dan weer acts as an adverbial phrase, putting it at the front triggers inversion (verb before subject). If you started the sentence this way, it would become: Dan weer is de hond rustig, dan weer is hij wild.

Why is there no verb or subject in the second half of the sentence (... dan weer wild)?

This is a grammatical feature called ellipsis, where you leave out words that are already understood. You can drop the subject and verb in the second part because they are identical to the first part (De hond is). Leaving the repeated words out makes the sentence punchier and more natural.

Can I use dan weer ... dan weer with verbs instead of adjectives like rustig and wild?

Absolutely. You can use it to describe alternating actions as well as states. For example, if you wanted to use verbs, you could say: De hond rent dan weer rond, dan weer slaapt hij (The dog runs around one minute, then sleeps the next).