"Comemos" is present and past. Even though it does not makes sense in English to say "Where do we eat on Sunday?", that is a correct word by word translation. We would say "Where do we eat on Sundays?" which is not the meaning in the source sentence. However, past tense has not been covered by the course thus far. You cannot expect beginners to know that "comemos" means "we ate" as well as "we eat".
They may have decided not to allow it, because the future tense can be used for anywhere in the future, whereas the limit of using the simple present in many languages is not for all future but for the near future with a time, just as the English present tense can also be used in this way "Where are we eating on Sunday?" . No, Portuguese is different from Spanish here, they do not use the present for future since the past has the same form and they do have a present continuous and near future (going to eat with the verb ir),
"Where will we eat on Sunday?" would be translated as "Onde nós comeremos no domingo?"
It looks as though even the continuous present, the near future and the form "I used to eat on Sundays" each have their own form in Portuguese which leaves "Where do I eat on Sunday?" and "Where did we eat on Sunday?" as the only acceptable alternatives. Yet, "Where do I eat today?" would be acceptable if you are looking for your seat to eat right now, but on Sunday if we are talking about the future in Portugese apparently they have another form for that. So my answer was perhaps not supposed to be marked correct.
Actually, "Onde nós comemos no domingo?" can be "where do we eat on Sundays? / Where did we use to eat on Sundays?" as well as "Where did we eat on Sunday?". Of course using "nos domingos" would make it clearer that is related to a habit, but the first also has this connotation because of the tense "presente do indicativo".
The most common way to express the future tense in Portuguese is to use the verb "Ir" (to go) in the present tense + the other verb in the infinitive (Onde nós vamos comer no domingo?). The present tense is also used, although not as much as the form above, but it's still more common than the actual future tense, which is rarely used.
But in this case, where the present tense and the past tense are the same, it can be confusing, so it's not used. An alternative way would be to use the informal expression "a gente", because it's conjugated in the 3rd person singular (Onde a gente come no domingo?), which has a different conjugation in the past tense, so you can't mix them up.
I hope that wasn't too confusing.
I have a question regarding the pronunciation: The voice always pronounces words that end with an o like they would end with an u. domingo ->" domingu". Yesterday I spoke to some people from Brazil and they told me that officially one pronounces it like an o in the spanish "yo hablo". Only in informal occasions like on the street people pronounce it with an u at the end. So shouldn't we learn the official pronunciation which we can use in e.g. business?
Thanks for the reply Lívia! Yeah Brazilians, keep confusing me! haha Everybody says something different and i don't know what to stick to. Yesterday three Brazilians said that they pronounce the o like an o, but one girl said the same thing like you (with rather an u). What should I stick to???? :D
I would say stick to both ;-)
Don't panic. I'll explain: in Brazil we have many variations depending on the region (just like every country in the world!). In the south, it is common to pronounce the final "o" and "e" like an "oh" and "eh" (closed, round "o" and "e" like in English "go" and "bend" (maybe, I'm not sure)). In most parts of Brazil, however, final "o" and "e" tend to become a short "u" and "i" (portuguese sounds for "u" and "i" here), except when they are stressed (in this case, the sounds may be more openned, like English "got" and "bed" and the accents will tell you how to pronounce: "ô" and "ê" for closed sounds and "ó" and "é" for openned sounds).
Besides that, we tend to say the final "o" and "e" so as to make it clearer. I believe it is just like in English, for example, when you say the article "a" that usually sounds like "ah" and when you want to say it clearer you say "ey" (I'm not sure if I could represent the sounds properly, but I hope you've got my point).
Finally, as Livsouza92 said, it is very important to know where to put the stress on the words. The given sentence has "domingo" and if you put the stress on the "min" you will be fine saying "doMINgu" or "doMINgo". Just to give an example with a very similar word, think about "dominó" (the game with flat blocks, just like in English). The "acento agudo" on the final "ó" makes you pronounce it like "domiNO" (with openned final "o" like English "got").
I hope it is a little bit clearer know.