"She drinks water."
Translation:Lei beve l'acqua.
I think that's just an Italian language convention, to put an article in front of some nouns. Like you would say "Capisco l'italiano" but you wouldn't translate it into English as "I understand the Italian." Such as "Io mangio il pane" means both "I eat bread" and "I eat the bread."
Mangi is "you eat" (informal), mangia is he/she/it eats, or "you eat" (formal).
This link should give you an idea of present tense conjugations:
Mangiare changes according to regular verbs ending in "-are"). Bere changes according to verbs ending in "-ere" (but is irregular so changes "R" to "V"). The link explains how verbs change according to the subject (who or what is doing the action).