It's a little difficult isn't it)) That sentence sounds indefinitely. Therefore, you'll get the follow-up question: О каком человеке? (who are you talking about?). In this case you should use the word "человечество". Mankind means человечество or человеческий род.
Note: A native Russian speaker would most likely say: я читаю книгу о природе человека (о человечестве it's no bad, too).
Those are different forms of the pronoun "'это", and to my knowledge they correspond to the gender of the noun which comes after it. этот - masculine. e.g. Этот студент. (This student) эта - feminine. e.g. Эта девочка. (This girl) это - neuter. e.g. Это письмо. (This letter) эти - plural. e.g. Эти газеты. (These newspapers)
Not exactly. If you say "15 человек" then yes it means "15 persons", males and females. But if you say "возле дома стоял человек" it almost always means a man (male).
One more thing. "Молодой человек" is 1) very popular address to young and not so young men; 2) boyfriend.
As for addresses Russian had "товарищ" or "гражданин" in Soviet era, not popular now. "Господин/госпожа" is used officially but not yet too popular in other cases. So most popular are "молодой человек / девушка / мужчина / женщина" depending on gender and age. Some people don't like when they are addressed as "мужчина / женщина" probably due to the fact that they don't look young anymore. But there is actually no more ways to address to them. Sometimes I envy english-speaking persons as they don't need to think too much about addresses and only have "you" not "ты/вы". It really simplifies communication.
okay thanks. Just to give some background info, if you like: https://en.wikipedia.org/wiki/King_Lear_(1971_USSR_film) It is the first Russian movie I ever watched. I do not have a transcript but I am pretty sure they translated fellow from Shakespeare to chelovek.
I believe this is the original line in question:
A very honest-hearted fellow, and as poor as the king.
I remember the scene and it's when Kent enters in the presence of the King in disguise, and he says: Chelovek--Chisniy Maliy.