"The girl wants to know everything."
Translation:Девушка хочет знать всё.
Not really... For a boyfriend a girl might use "парень" or, less informal, "молодой человек". Calling a boy "парень", though, is rather colloquial, so in more formal situations "молодой человек" or a slightly outdated "юноша" is used by people like teachers.
"Outdated" in the meaning that I am 28, and do not plan on using юноша as I become even older—though I am already at the stage when boys from high school seem infinitely younger.
When handwaving about a young fella you met the other day, "молодой парень" is also not unheard of.
"Мальчик" is a little boy. Women tend to use "мальчики" towards males at their workplace, too (especially those younger) but I do not find that much to my liking.
It is safe to say that there is no real "absolutely neutral" way to refer to a late teen/young adult boy in modern Russian. Парень seems slightly informal to me, молодой человек is OK for a boyfriend but a bit formal, чувак or пацан are WAY informal (dude, chap, lad).
Just to give some context, when I was in Russia in my early twenties, anyone who needed to get my attention would call out "молодой человек," never "мальчик." Мальчик is really only for very young boys (preteen and below, teens are all "хулиганы" (I kid)).
"Чувак" and "пацан" become much more common when you get on informal terms with people, and you'll hear it a lot when people are telling stories about some random guy ("Такой чувак пришел...").
For a collective group, "ребята" is used roughly the same way we use "guys" in English.
This is the Russian course. We generally do not translate этот as the and vice versa.
In the course of English you occasionally have этот to remind you where the definite article should be used. This course, on the other hand, is aimed at native or fluent speakers of English, who rarely forget to use articles.