Translation:The day before yesterday was Sunday.
The emphasis is perhaps different. Arguably your sentence is more likely to answer the question "when was Sunday" instead of the question "what was the day before yesterday".
Assuming that "星期天是前天" is a valid Chinese sentence, I wouldn't really advise using your suggestion to translate Duo's sentence.
Chinese native speakers?
Native here. It is good that the word order matches, so that the sentence can be understood more quickly. In my opinion the order of this type of sentences (“noun A = noun B”) is not essential in Chinese even if there is a context, unless we are demanded to use a specific order.
"The day before yesterday was a Sunday," should be accepted because the "a" doesn't change the meaning in English.
It is confusing and with the 很which also is sometimes translated and sometimes not. Duo has to learn better English and Chinese!!!
To be clear, though, "很" doesn't come into play with this particular sentence.
"很" (literally "very", but sometimes described as a sort of copular filler similar to "to be") is used with an adjectival subject complement, whereas "是" (literally "to be", in whatever form of the latter is appropriate) is used with a nominal subject complement.
Using “is” is also correct while calculating the day of the week, isn't it?
For some reason when we refer to a future day (e.g. the day after tomorrow) we can use the present tense but when we refer to a day in the past we almost invariably use the past tense. (At the moment I can't think of a scenario in which we would use the present tense to refer to a day in the past, but I imagine one could be thought of.)
Edit: I've thought of a scenario. If you're talking generally about dates and looking at a calendar, you might say "the day before that is Sunday", which is an abstraction disconnected from the real time that we're now experiencing. However, when we're currently experiencing the day called "today", and talking about the day before "yesterday", those are "real" days in our timeline as we've experienced it, and so we mark our reference to past days with the past tense.
No, because you are talking about the day before yesterday. The day before yesterday is only Sunday if today is Tuesday.
I'm not really following your second sentence (in terms of the semantic logic you're trying to present with respect to the verb tense). I guess as an abstraction it could support the present tense, but generally when we talk about "yesterday" and "the day before yesterday" we don't say "is", we say "was".
Someone please help. "The day before yesterday was" would be "前天是" not "前昨天是"?? Thank you!
Yesterday is 昨天 and the day before yesterday is 前天; 2 days before yesterday is 大前天. We don't say them like the day before xxx literally which sounds awkward in Chinese.
For the day before a specific festive or memorial day we say xxx 前夕, e.g. 圣诞前夕 Christmas Eve.
For the day before an ordinary day, we say ~ 前一天, e.g. 我们出发前一天 The day before we set off (a trip). (This is only an instance and other variations exist.)