Translation:¿Cuánta agua necesitas?
"Agua" is femine (and not masculine). It takes "el" in the singular because of a clashing of vowel sounds, and "las" in the plural.
I'm not sure for un/una, but for other things like "esta" and clearly "cuánta" it goes to the feminine form (i.e. I can't describe the sounds to differentiate for myself but it is roughly the way it works)
My Spanish teachers always taught us that aqua, like idioma, was from Latin and was one of the few exceptions to the "words ending in A are feminine" rule .
Why, though, when I tried this again, got the ¿Cuánta agua part right, and ended with "necesita usted?" did it object to my using the formal form of "you," and mark it wrong?
There is NOTHING about "how much water do you need" that either indicates, or would tend to MANDATE the sentence MUST use the informal "you" conjugation of "necesitar".
THAT should probably be fixed, or at least add an indication of which form, formal or informal, or when the singular or plural, is required. In English, "you" can mean any of many things and it's ambiguous. Formal or informal, (or familiar and polite, if you prefer,) and it's also both singular and plural. Spanish has at least about a half dozen words that all translate as "you" and are NOT interchangeable:
Tú: you (sing. fam.) Usted: you (sing. pol.) Ustedes: you (pl. pol.) Vosotros: you (pl. exclusively masculine, or mixed m/f, fam.) Vosotras: you (pl. exclusively fem., fam.) - Yeah, I know not all dialects use vosotro/as -
There may be others... I am not as yet fluent en español... not even close.
I also used "necesita". But it does not accept it yet.
I made my report. 25/august/2018
Did not accept '¿Cuánta agua necesitáis?' and it isn't giving me the option to report it - 18 July 2018.
because you are asking "How much water do you need?" not, "How much do you need water?"
This answer is incorrect Agua is masculine and we should use cuanto
Being used with el does NOT make a noun masculine. USUALLY el IS used just as you've indicated, BUT in this case, (and the case of all feminine nouns beginning with either a, because la agua would sound like "lahgwa" thanks to something called "synalepha," (or "sinalefa" en español,) the blending together of a pair of adjacent vowel sounds, into the sound of a single word, even though they're written as two, SEPARATE words. Or something like that. This is just one of those things in Spanish. Duolingo isn't wrong here, although an explanation of WHY when it tells you you're wrong, (I'd forgotten agua is NOT in fact masculine, despite the fact that it takes EL and not LA, which is how I ended up here,) WOULD be helpful.
For future reference, there's another thing you'll likely find confusing, and that's consequence of the "ma/pa/ta rule". The generality that in Spanish, words ending in "a" are feminine does NOT apply to most (if not indeed, all,) words ending in a, if the previous letter is m, p, or t... as in ending in ma, pa, and ta, because generally, those words are Greek in origin, (IIRC,) and either because they're masculine in Greek, or on the other hand, maybe "just because," (can't recall for certain why,) in Spanish they're masculine. So "a map" is not una mapa, but rather UN mapa. (You may consult Google Translate on this one... https://translate.google.com/#auto/es/The%20map).
In some cases, confusingly, it can be either. Papa is a fun example. La papa = the potato. El papa = the Pope. You probably don't want to confuse those two. People might look at you funny if you said you thought the Pope tasted good, especially baked with butter, sour cream, and chives... mmmm.... baked potatoes... :-) They might also think it odd if you wrote or stated that you wanted to meet the potato, get its blessing, or get a dispensation from it for something.
Then again, there's "radio." Looks like a masculine word, ending in o, right?
But you must use either el or la with it, depending on whether you're talking about, for example, radio as in the electromagnetic waves propagating through the air, carrying perhaps voice or music information modulated onto it in some way, or if you're referring to the device that picks up those waves, and turns them into sound.
Also, el radio can mean the radius of something, like a circle, or (I think,) the bone in the arm that isn't the humerus or the ulna. Or a radioactive chemical. (Ref: http://buscon.rae.es/dpd/?key=radio&origen=REDPD.) This is THE reference for what IS Spanish. Sadly, it's IN Spanish, so it is a little tough for someone not yet fluent in the language to read. But I think I got all that right.)
I agree with WasHimk. I understand about el agua being feminine (I learned this when I tried to say "agua frio" and was corrected. It s/b "agua fria". But if they use the "el" to avoid the clashing a's, why not use cuanto agua?