"You are growing really fast."
Translation:Te kasvatte todella nopeasti.
I do not want to find the correct answer by process of elimination, i.e. the other two sentences do not contain the right words or verbforms. What I want is to understand how to differentiate whether singular or plural is meant! (Never thought I'd say this but... Finnish is easier in this case!)
Nope. "Nopeasti" is only an adverb and "nopea" is only an adjective. But it should be kept in mind that English and Finnish use adverbs and adjectives differently sometimes. For instance, the phrase "you are correct" uses an adjective in the predicative, but the Finnish translation, "(sinä/te) olet(te) oikeassa", uses an adverb and thus has an adverbial instead of a predicative.
I said "oikein" instead of "todella", which means much the same thing, doesn't it? It's too bad the system can't credit me for knowing words that haven't been part of the lesson yet.
If there are any native speakers here - would there be any significant difference in meaning between "todella nopeasti" and "oikein nopeasti"?