The imperfect refers to things that happened continuously in the past. For example, the program "No sabía que estaba embarazada" is called "I Didn't Know I Was Pregnant" in English, which means "I used to not know I was pregnant" because knowing/not knowing how to do something is a continuous action.
Some verbs change meaning in the Preterit, and saber is one of them. Look at this link: http://www.spanishdict.com/topics/show/63:
saber | to know (Imperfective) | to find out (Perfective)
Imperfective Aspect- Present tense, Imperfect tense, Progressive tenses, Future tense, Conditional tense
Perfective Aspect- Preterit tense, All perfect tenses (present perfect, past perfect, etc.)
"La víctima no supo nadar" would translate "The victim didn't find out how to swim".
Saber is used with abilities and skills that are learnt like Conducir (to drive), pintar (to paint) etc. To say you know how to do something, one would use Saber + Infinitive.
The use of saber in the duo sentence means 'to know how', thus: La víctima no sabía nadar=
La victima=the victim
no sabía= did not know how
nadar= to swim
Here is a reference: (look at C under saber) http://www.spanish.cl/Vocabulary/Notes/Conocer_vs_Saber.htm
That is only the case for the word "didn't" because it is already conjugated. If there are other negative words; like "never", "nobody"; before "used to"; it doesn't make sense, grammar wise, not to conjugate that verb in the past because those negative words aren't verbs.
For example, "did you use to swim with your girlfriend?", but "I never used to eat paper when I was a kid".
I think the main reason why it didn't accept their answer is because they used "never", which would be "nunca" in Spanish, instead of "didn't". In that case, it would be "use to" like I mentioned previously.
In this thread "know" is a stative verb (state verb). There are 2 basic rules for stative verbs.
- Generally stative verbs are not used in the continuous form
- Some, not all, stative verbs can be used in the continuous if there is a need to show the temporary nature of the verb
Unfortunately your translation breaks both these rules thus your question itself is mute
I don't know about 'to know how' being a static verb, but the Spanish past imperfect indicative is used in several ways and one of them is setting the stage for another event such as:
Yo leía cuando entró mi papá. I was reading when my papa entered. (note that "entered" is preterite) The Duo sentence as written doesn't seem to be setting the stage, but I think it might be possible. Also the imperfect indicative is also used to describe something in the past. This sentence might fall into this category. We can translate into English the past imperfect indicative when we are describing an event which does not have a definite start time or end time. It is like a general statement, such as:
I was working in the agency during the day. Trabajaba durante el día.
Your reply may be correct but has no bearing on my comment above, which is a direct reply to soderdaen post ref. "The victim was not knowing how to swim"
In other words the sentence "The victim was not knowing how to swim" is not correct in English, thus can not be used as a translation of the Spanish.
The reason it is incorrect is that the primary verb is a stative verb and is subject to a different set of rules than action verbs such as "run"
No. In Spanish, to say "to know how to do something", you use the construction "saber + infinitive". Literally: to know to do something.
Sé leer Sabes caminar Saben descargar el documento.
etc,... It's one of those things that are just different across languages. Direct translations aren't always correct.