Translation:It was three days ago that we arrived.
It's not a passive English sentence; it's a so-called "emphatic" one (which is just to say that it provides a certain emphasis or focus not present in the simple version of the sentence). This particular kind of emphatic structure is called a cleft sentence:
Its appropriateness would depend on the context, but it can be used to elucidate the "是...的" structure:
As for the passive voice, which is completely different, it's explained in the following article, which also touches on the misuse of the term:
The process of the movement of the underlying subject to a place farther back in the sentence, while replacing it with the 'dummy subject' can be referred to as extraposition. The resulting cleft sentence puts emphasis or focus on X in 'It was X, that ....' As others have noted, this is a common way to translate the Chinese 是的 structure.
I think it is too do with the 是... 的 sentence construction. Emphasis is placed on what ever is between those two words. In this case arrival 3 days ago as opposed to any other length of time
It's not a passive. To make a passive you need to make the object of the verb the subject and this sentence has no verb with an object. I would say this is in a narrative style like you might read in a novel. So I agree it's a strange word order to pick for a course like this.
Then again, the course assumes we know English and we're studying Chinese. But that doesn't really justify it since it makes a worse match to the Chinese.
It's about the usage of 是 and 的 together. Because the 是 is there, the sentence would be incorrect without the 的. 我们三天前到了 would also be correct.
"到" = "arrive"; "來" = "come". The nuance is similar in both languages. "Come" typically implies movement towards the speaker's current location. "Arrive" doesn't. Depending on the context, they may or may not mean essentially the same thing. Here there's no clear context, so you want to stick with the literal translation.
It's not too casual. Often there are multiple correct translations between a pair of languages because a term in one equates with two or three terms in the other. Compare:
- 带 give / take
- 回 return / come back / go back / return
- 回家 go home / come home / return home
How about: It's three days ago that we arrived? In the sense of it is (now) 3 days since arrival.
Although I might use 'is' (or 'has been') with 'since,' I would use 'was' rather than 'is' here with 'that.' The difference for me in English is whether your point of reference in terms of time is the present or the time at which you arrived.