You just need to know the infinitive (in this case,
essere) and put it into a conjugator:
lei=she Lei= formal you
i hope the following will explain the differences and put an end to the discussion.
As if learning a new language weren't difficult enough, Romance languages such as Italian have separate forms of address in formal versus informal situations. This How-To has quick, step-by-step instructions on how to use the formal and informal subject pronouns tu, voi, lei, and loro.
There are four ways of saying you in Italian: tu, voi, Lei, and Loro. Tu (for one person) and voi (for two or more people) are the familiar forms, used only with family members, children, and close friends. Use Lei (for one person, male or female) and its plural Loro in more formal situations to address strangers, acquaintances, older people, or people in authority. Capitalize Lei and Loro to distinguish them from lei (she) and loro (they).
That's a huge oversimplification: you'll be corrected just as well if you use Lei with people who expect tu. You should generally use Lei with people older than you or people you have to show respect to, and you definitely have to use tu with children and teenagers (in this specific sentence, you would never call a "ragazzo" Lei), but in the middle there's a whole lot of social awareness and reading the situation: tu can be friendly or rude, and Lei stiff or polite. I remember my English teacher in university asked us why the rest of the staff used Lei with her in writing and tu in person. In front of a computer people generally use tu (the software does too), and over in the English course there's plenty of people that don't understand why Duolingo would translate you with Lei. As for why the course uses tu more than Lei, I think it's just that it's much less confusing and ambiguous.
il is one of the ways to say "the".
su is a preposition that more or less corresponds with the English preposition "on" or "up".
Rules for the definite article:
Rules for the indefinite article:
The prepositions combine with the definite articles:
As a verb, it's the second person singular present tense of "to be" (you are).
As a number, it's 6.
sei is the present tense "to be" conjugation for the singular "you":
è is the present tense "to be" conjugation for "he" or "she":
uno are like the English "a/an" but follow different rules:
io = I
tu = you (singular)
lui = he
lei = she
noi = we
voi = you (plural)
loro = they
io sono = I am
tu sei = you are
lui/lei è = he/she is
noi siamo = we are
voi siete = you are
loro sono = they are
un = a/an (masculine)
una = a/an (feminine)
ragazzo = boy
ragazzi = boys
ragazza = girl
ragazze = girls
Put all that together for this lesson:
Tu sei un ragazzo = You are a boy.
I hope that answered your question, because you did not specify what you don't understand.