"Half the students in the class are thin."
Translation:Половина учеников в классе худые.
For largely the same reason that makes us say 'the other half of the pupils are fat', but 'the other half of the lemon has been eaten by my cat'. Collective nouns can function either as subjects of the sentence in their own right (in which case the verb agrees with the collective noun - this is called formal agreement) or as modifiers of a plural noun, explicitly stated or implied (in which case the verb agrees with that other noun - this is called notional agreement).
There are a few ways to tell which kind of agreement to use in each particular case in Russian.
You strictly use the formal agreement if the collective noun stands on its own, with no subordinate nouns or any other qualifying words at all.
E.g. "Половина была против".
"Большинство проголосовало за".
Also, it is recommended to use the formal agreement if the collective noun only governs a noun or several nouns in singular, with no plural nouns amongst them.
E.g. "Половина нашей команды болеет".
Now, for the cases when it's better to use the notional agreement.
When you're stressing the fact that every specific person out of the people mentioned collectively is doing the thing in question somewhat separately from others, acting as an agent in their own right. This is also something you can see in BrE when words like 'team' are treated as plural nouns. And this is partly what's happening in this specific Duolingo sentence: every single pupil is thin in a unique way, and they're thin for their own reasons, without conspiring with others. Hence the plural.
Would also work if this sentence had a verb: "Половина учеников в классе мало едят".
When the predicate includes a plural noun or a plural adjective along with (or instead of) the verb. This is also what's happening here.
E.g. "Половина лимонов были гнилые".
If the plural noun has a qualifier in the form of a participial construction, or a subordinate clause starting with "которые".
E.g. "Половина людей, болеющих ветрянкой, становятся леопардами". (Note that the participle is also in plural.)
"Половина людей, которые держат дома кроликов, называют себя Алисами".
If there are several verbs instead of just one.
E.g. "Половина учеников в классе бездельничают и не слушают учителя".
Once again, I mourn the fact that DL won't let us "save" specific comments. This is gold.
"Тонкие". Aye, this one's mostly used when referring to inanimate objects.
You can, theoretically, call a specific person "тонкий", but only one at a time; and even then it would sound somewhat rural and old-fashioned.
Thanks. Is there another word that would be synonymous with slender, lithe or willowy (positive words for being thin)? Maybe I'm wrong (or olde), but for some reason I associate худой with being sickly or skinny in a less aesthetically pleasing sense. Yes, my first association with the word тонкий ис тонкая рябина. :)
Well, you are in fact right about the original meaning of the word. Худой is related to the word худо - evil, bad luck. Худой means bad, ill, vicious or torn / having holes in it. In modern language these meanings are mostly forgotten and are hardly ever used, in general this word is not perceived as negative anymore. Стройный is usually used for complimenting a slim and fit figure. Тонкий is somewhat poetic and can be used more like a metaphore. Худощавый is more about the bone structure, an asthenic person. Тощий is negative, usually it's sickly looking person.
Стройный/стройная should do the trick. Although using it to refer to a male (and probably a child) would most likely connote the same sickly thinness; and when directed at groups of people it sounds almost as weird as "тонкий". But the same can also be said of the English counterparts, so no surprises here.
"Все они стройные ребята" doesn't sound weird at all; moreover, it's a compliment as it's somewhat similar to "graceful". Speaking of a woman "стройная" is close to svelte.
Hmmm. Стройный/стройная doesn't mean 'well-built'/'well put-together' or even 'strong'? Wow, I have some misconceptions then. In American English, slender is generally positive for both genders, lithe or willowy is positive for women, thin is neutral or contextual, skinny is veering towards negative unless you admire anorexia, and scrawny is negative.
Nah, "стройный" is strictly 'slim', implying an aesthetically pleasing silhouette. The word being a close relative of "строить" is a false friend.
Well, I can only say that, being a male, I've been called 'slender' by many British and Irish old ladies in my life, and then they'd usually poke me in the belly and offer some more cake - so I may have brought some of my personal biases into this, aye. Sorries. ;)
Speaking of humans, one can say, "Он/Она человек тонкий" meaning "He/She is a person of fine fiber // of a spiritual type". "Тонкая материя" has two meanings: (1) fine fabric, and (2) sensitive/subtle matter.
No. Normally Russians would say, Полкласса худые. Or Половина учеников в классе худые, which sounds quite formal. A high school student is called ученик; the word студент only refers to a university/college student.