"You read the book."
Translation:Tu leggi il libro.
Tu is singular. Leggi is singular. Tu leggi. Voi is plural. Leggete is plural. Voi leggete. Think of old english (in some regions also nowadays). Thou read is singular, you read is plural. In English the verb doesn't change in this case (but it does e.g. in the 3rd person. He/She reads).
Are you asking about the fact that the articles change with the noun? "La" is almost always feminine (there are a few exceptions), "il" is masculine, and "lo" is used for masculine singular "s-impure" words. ("The joke"="Lo sbaglio" for instance.) The plural of "la" is "le", the plural of "il" is "i", and the plural of "lo" is "gli". Almost all words ending in "a" are feminine, "o" or "e" are masculine.
If you are translating FROM English, into Italian, and you don't know from the context if it's one person or many people, then DL will accept either translation (single "you" or plural "you") but make sure you conjugate correctly. "Tu leggete" is wrong because "tu" is singular, and "leggete" is plural. However, translating FROM Italian should be simpler. The rules for singular versus plural are quite straight forward....unlike English....or German....