"Gli" is the plural form of the masculine definite article "lo." Lo, l' and Gli are used only when the noun the definite article is modifying starts with the sound of "s impura (s + consonant), (lo Stato), z (lo zio), gn (lo gnomo), sc (lo sciocco), pn (lo pneumatico), ps (lo psicologo), x (lo xilofono), and with i semiconsonantica (semivowel i) (lo iodio)." — http://italian.about.com/od/grammar/a/italian-definite-articles-il-lo.htm

Yes. "Le ragazze" is feminine plural. http://www.oneworlditaliano.com/english/italian-grammar/italian-definite-articles.htm

Its plural, I think so

If the root is "Bev" then: Bev-o ending is used for I Bev-i ending is used for you Bev-e ending is used for she/he/it Bev-iamo ending is used for we Bev-ete ending is used for you all Bev-ono endinf is uaes for they

Hipe this helped

You can use both, it isn't necessary to use "loro".

You see, 'bevono' means they drink. Loro bevono really means they they drink

That would be for very young children or babies.