Well, "could" could be either imperfect or perfect past.
The main difference is that the PP represents something that happened once in the past, while the IP indicates an ongoing action in the past.
Some people also say that PP should talk about unusual happenings while IP determines habitual/common facts (but this isn't really a 100% accurate science xD).
Eu comi em casa = once, probably before meeting this person I'm talking to, I ate at home.
Eu comia em casa = I used to eat at home, but not anymore. I'm pretty sure in English you would never say "I ate at home" when meaning to say "I used to eat at home", but that would be the literal translation
BUT! "Used to" can't always be translated to IP, because sometimes it has another meaning, like when you use it with "to be" and it becomes "acostumar": I'm used to this = Estou acostumado com isto.
So, here, since we don't have any context, you could use PP and it would be correct (Eles não puderam ver), but I'd say that "Eles não podiam ver" is what comes to mind first. Both sentences sound weird though so it's hard to understand which one we're supposed to use :\
They are synonyms, for sure, but they're NOT the same:
They weren't able to see = Eles não eram capazes ver (were not = denial = could translate to não podiam/não conseguiam)
They were unable to see = Eles eram/estavam incapazes de ver (there's no denial in this sentence - so we can't translate it to "não podiam/não conseguiam")
It is pretty tricky (and wouldn't make a difference in conversation), but I think DL is right to not accept synonyms because that does help people learn the differences C:
In certain regions of Brazil, the syllable "di" — as well as "de" when it's a non-stressed final syllable (and more rarely/possibly in the middle of words, but always non-stressed), — is pronounced as
The very same thing occurs to the syllables "ti" and "te" in similar contexts, pronounced
There are Brazilian regions, though, where those syllables are simply pronounced as /di/ and /ti/. Both ways are correct, it's just a matter of accent.
The hints are wrong! "can" (to be able) is an irregular verb in English. The past tense of "can" is "could". However "can" as a process for preserving food is a regular verb and the past tense is "canned". If English is not your native language you might well be confused, but Duolingo should know better.
For me, "They couldn't see" means that for some reason, at that given time, they weren't able to see. They were blindfolded or the lights went out.
Now, "They used to not be able to see" sounds like they used to be blind and now they can see. If this is true, the sentence would translate to Eles costumavam não poder ver.
If someone is really trying to use your sentence with the same meaning as the DL sentence, then it would be translated as "não podiam ver"... But that's kind of a longshot xD
However, "podiam" is one way proper to translate "be able to". If you want to use "used to" along with that, you need to make sure that it fits the Imperfect Past tense (an ongoing action in the past):
I used to eat at home = Eu comia em casa
You can also translate it to "Eu costumava comer em casa", but no one says it like that because we're used to use "comia" in cases like this.
Can somebody explain the difference between poderei and pude and podia? I've seen all three used to say "i could" so far and a bit confused....
I think i understand that pude is past perfect used for events that are specific and complete. And podia is imperfect- used for ongoing events. Can anybody confirm? Thank you!