"How efficient are the horses?"
Translation:Quanto sono efficaci i cavalli?
In this case "quanto" doesn't have to agree in gender and number with the noun, since it's not an adjective referring to it. "Quanto + adj" is equivalent to "how + adj" in the English construction. However, when translating "how much/many", you'd have literally "quanto tanto/a/i/e" which becomes just "quanto/a/i/e". That's the only case you have a declensed form of "quanto".
I think we are looking at subtly different meanings in the two languages.
In English, confusing efficient and effective is a classic schoolchild's error, of which English teachers are heartily fed up. "Being effective is about doing the right things, while being efficient is about doing things right". Effective is whether something produces a desired effect. Efficient is how well the effect is produced, and in theory can be calculated as units output per unit input.
I think Italian has a different logic, which I find hard to figure out. Dictionary examples suggest that efficiente applied to a person (or a horse or a machine) translates to "efficient", but to non-actors (e.g. a process, a method, a remedy) it translates to "effective". Duo doesn't follow this logic.
There's another Italian phrase for the English "efficient": di buon rendimento. The noun means yield, performance, efficiency or duty according to context. Unfortunately I don't think there's a related adjective.
I confirm this. Efficiente is more like productive, like a horse which doesn't eat too much but still runs for a long time before getting tired. "cavalli" is also a commonly used short form of "cavalli vapore", which would fit better than my example with horses in terms of an engine efficiency.
John I am sure that you are right but I thought "come" was more appropriate because the sentence is about the efficiency of horses and not motors . With motors the response would be perhaps, that the motor is 70% efficient, in which case quanto is clearly the correct interrogative. Whereas, with horse when talking about their efficiency we would not describe it's efficiency in numerical terms but rather in the general attributes of the horse ? What do you think ?
"How" = come for most things, but not for quantities. "How much/many" = quanto/i, obviously. Less obvious but logical is when the "much" is replaced by an adjective of degree like "long", or "effective" as here. [If Duo means "efficient" the answer should be efficiente. English speakers often get them mixed up too.]
There's one context in which come / quanto are interchangeable, and that's as an intensifier in an exclamation: "How effective the horses are!". In Duo's example there is no exclamation mark and the English word order is wrong. Even if these were present and correct, your Italian word order is wrong: it should have been Come sono efficaci i cavalli!.
Italian word order is pretty fluid, but here quanto is an (interrogative) adverb, and you don't really want to separate it from the verb or adjective that it modifies. It's a general 'rule'.
References in grammar to quanto è specifically seem to be rare, but the Treccani dictionary says "Come avv[erbio]: ... modificando un agg[ettivo]: quant’è lungo il cammino?. This order makes for easy additions, such as ... più lungo ..., and compound tenses. If you think of the noun first, it's perfectly OK to say I cavalli, quanto sono efficaci? I'm not sure about ... sono quanto efficaci?.
On Earth we would answer it variously, such as "very, fairly, not very, not at all, 40% (meaning power output divided by power input)", etc. How about your planet? :-) If you ignore Duo's literal translation by dropping "the", you have "How efficient are horses?" - a genuine question of biophysics or agriculture.
In addition to all valid questions about where 'sono' should go in the sentence, as discussed here, I would kindly point out to DL that, just like 'efficient' and 'effective' have significantly different meanings in English, I frivolously assume the same semantic distinction applies to 'efficiente' and 'efficace'.
So, DL, please, respect the correspondence.
Why is it "quanto" and not "quanti" here when we are talking about horses (plural)?
And why is "efficaci" AFTER "sono"? I put "quanti efficaci sono i cavalli" (thinking it literally translates to "how efficient are the horses" and was marked wrong, but I will try to just change "quanti" to "quanto" next time and see if where I put the adjective will be accepted that way too or not.
As a language teacher, language is alive, it changes meaning with acceptance. Just because something is right grammatically according to the Grammar Book doesn't mean that one has to use it that way. Dialects account for so many differences, y'all. For me, efficient means things are done in a minimum amount of time, while effective means things are done within a minimum amount of effort.