Yes, det also means "it" depending on the gender of "it" (but you'd have to change the adjective to match the gender)
Yes, when an adverbial phrase (something to do with time or location) moves to the front of the sentence, then the verb has to stay in the second position. So it is "Vi spiser mad i England" (We eat food in England) but if you put the adverbial phrase at the front it would be "I England spiser vi mad" (In England we eat food)
What is the difference between "den" / "der" / "de" / "det"? When have we to use them?
I am not a native speaker, but I have been studying for several years now. I believe den is used for “it” primarily with common words, “Den er ikke nødvendigvis en fisk”, and det is used with neuter words , such as: Det er ikke privat, det er personligt, although I notice duo seems to defer to det frequently in situations where there is no indication of whether the “it” in question is a neuter or common word. De is plural. As in De er trætte, They are tired. Der means there; Der er alternativer. There are alternatives. Hope this helps.