A previous sentence in Duo was "He could not have gone to school" The translation given was "Él no puede haber ido a la escuela" which could also be translated as "He cannot have gone to school" So I'm thinking this sentence could also be translated as "She couldn't have been sick"
The problem is that most English speakers do not use all of the tenses In normal conversation. The sentence "she could not have been ill" can be written
(Previously) she could not have (in the past) been ill.
This is talking about a situation that happened in the past of the past. I know it sounds like semantics but it really does make a difference in the meaning.
I'm not a Spanish speaker, so I don't know about this "past of the past" situation that you are talking about.
But "she cannot have been ill" is improper English. The only way to say this sentence in English is "she COULD not have been ill."
Just take out the word "not" and it is easier to see that "she can have been ill" is grammatically incorrect. There is no such thing as "can have been."
(Yes, I'm saying that Duolingo is wrong here and I reported it to them just now.)
I've noticed past tenses are being questioned a lot on this site. I agree THIS sentence was wrong, but for some reason lots of people have issues with Past Perfect tense. This is what I mean by "Past of the Past".
"I went to go buy it, but had not stopped to get money."
The time in which I was "buying it" happened in the past. The time in which (or rather the time I didn't) get money was prior to the time of "buying it."
In English we do not typically make the distinction during conjugation and instead add additional information to the sentence.
"I went to go buy it, having previously stopped to get money."
It was probably the wrong time to bring this topic up. I kind of wish that Duolingo would identify, at least in the discussion section, which topic they were trying to teach. Was this a test of the use of "Haber"? Was this a test of the use of Simple Perfect, or Past Perfect?
I just talked to a co-worker about this question, he is new to Duolingo. He said that this sentence sounded correct in a specific context.
"To receive insurance, she cannot have been ill in the past 6 months."
Yes, you say "She could not have been" but as she is currently seeking insurance right at this moment, the test of her qualification must happen now as well. At this point in time, if she has had any illness, today she would not qualify."
Again, its a bad sentence but it gives us a good situation to critique our knowledge of grammar in both our native and foreign tongue.
Skepticalways, "she CAN, too, HAVE GONE to school with purple hair!" isn't proper English. There are better and proper ways to say your example sentence, depending on the exact situation. For example, "she can go to school with purple hair," or "she is allowed to go to school with purple hair" or "she could have gone to school with purple hair."
Robert, Not talking about the verb "can" that means "to preserve cans or jars of food," but the auxiliary verb - I think it is confusing people. In a similar construction, what if you were saying, "She is not lying (as she tells you about her school day); she CAN, too, HAVE GONE to school with purple hair! Her school tolerates such things." This example sentence would demonstrate that she can have done something (It IS possible that those events HAVE happened).
It would mean an entirely different story if you said, "She could have gone to school with purple hair - she was able to, and/or permitted to, but she did not, because she did not want negative attention from the teachers."
"Can" is used with other verbs, like can DO, can BE, can GO, etc. It seems to be a tricky verb, different in its conjugation. I can (today), I could (yesterday), but you canNOT say "have/has+past tense" for its present participle, because that would be, "has/have could"! You need to switch the order around and say "could have." But you cannot do the same for the past participle, because we don't say, "had could, OR could had"! I think you would have to switch ALL participles to "have been able to, had been able to, will have been able to."
Anyone think differently?
At least one reason is that there are two verbs next to one another. The first will be conjugated, the other will be in the infinitive (dictionary form). I recommend reviewing the skill named Activ2.
Just as a side note if you've tested out of any skills, since the implementation of Crowns, we've been seeing several higher level learners (so, lvl 17-25ish) with confusion about things that were taught early on in the course and then regularly revisited throughout. Usually, this is because folks have used the Test Out feature. I recommend not to test out of skills unless a learner had prior Spanish instruction before joining Duolingo and was confident in that skill before arriving. The current Test Out exam is too easy and is not an accurate indicator that someone is ready to bypass lessons. (Duolingo is always fiddling with things so, the test will likely change several times in the future. Hopefully, it will become considerably more difficult.) I don't mean this to be insulting. Rather, just a heads up if you are testing out of Crowns and skills that your language studies would be better served by not doing that. :)