Verb second: inversion

In a Danish main clause the finite verb is always the second element. If the sentence starts with the subject, the order is as in English: Jeg tager toget. If anything else comes first – a time or place phrase, an adverb like så, nu, derfor, heldigvis, a question word or a whole subordinate clause – the subject moves behind the verb (inversion):

• I morgen tager jeg toget. (Tomorrow I'm taking the train.)
• Hvad laver du? (What are you doing?)
• Når jeg har fri, tager jeg toget. (When I'm off, I take the train.)

The conjunctions og, men, eller, for don't count as an element, so straight order follows them: Jeg er træt, men jeg tager toget. In og så and og derfor, så or derfor comes first in its clause, so you invert: og så går vi.