Gisteren hebben wij over het ongeluk gesproken.

Yesterday we spoke about the accident.

Breakdown of Gisteren hebben wij over het ongeluk gesproken.

• wij: we
• spreken: to speak
• gisteren: yesterday
• over: about
• het ongeluk: the accident

Questions & Answers about Gisteren hebben wij over het ongeluk gesproken.

Why is gisteren placed at the very beginning of the sentence, and why does the verb hebben come before wij?

Dutch uses a V2 (verb-second) word order in main clauses. When you front a time adverbial like gisteren, the finite verb (hebben) must occupy the second position. That pushes the subject pronoun (wij) to third position. So:
• Position 1: gisteren (time)
• Position 2: hebben (finite verb)
• Position 3: wij (subject)

Could I also say Wij hebben gisteren over het ongeluk gesproken? Is there any difference in meaning or nuance?

Yes, you can. Keeping the subject first gives you the neutral SVO order:
  Wij hebben gisteren over het ongeluk gesproken.
The meaning is identical. Fronting gisteren only adds a slight emphasis on when the conversation took place.

Why is gesproken at the end of the sentence?

In Dutch perfect tense you form a “split verb” construction: the auxiliary (hebben) goes in second position, and the past participle (gesproken) comes at the very end of the clause. That’s standard for all compound tenses.

Why do we use hebben as the auxiliary? Could we ever use zijn with spreken?

Most Dutch verbs take hebben in the perfect. Only a small group of verbs expressing movement or change of state (e.g. gaan, komen, veranderen) use zijn. Since spreken is neither movement nor a change of state, you always use hebben.

What exactly does the preposition over mean in this context? Why not van or om?

Here over translates as “about” in English, indicating the topic of conversation.
• van often means “of” or “from” and doesn’t convey “talk about.”
• om has other uses (e.g. om acht uur = “at eight o’clock”) and isn’t used for “about.”

Why is there a definite article het before ongeluk? Could it be een ongeluk?

Use het ongeluk when you refer to a specific accident already known to speaker and listener. If you wanted to speak of any accident in general, you’d say over een ongeluk gesproken (“talked about an accident”).

Can I replace wij with we? Does that change the register or meaning?

Absolutely. we is the unstressed, informal form of wij and is very common in speech. wij feels slightly more emphatic or formal. Both are grammatically correct.

Do I have to include the subject wij/we in Dutch, or can I omit it like in Spanish or Italian?

Dutch is not a pro-drop language. You generally need an explicit subject in every finite clause, so you cannot omit wij or we here. Only in certain very casual ellipses might you leave out a subject, but not in a normal statement like this.