FIRST.
The formal you in Spanish translates into usted. So this is the subject pronoun for the second person singular (formal form). We use it when addressing someone older, a person you don't know, your teacher, your boss, etc. Thus, you use usted to show respect to anyone. Therefore, the sentence becomes:
Usted necesita estudiar para la clase de inglés
SECOND.
The informal you in Spanish translates into tú. This is also the subject pronoun for the second person singular (informal form). We use it when addressing someone you know well. You can use it to speak directly to children, close relatives, friends, peers, or pets. Therefore:
Tú necesitas estudiar para la clase de inglés
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See that the conjugations of the verb necesitar aren't the same for usted and tú. For usted this conjugation is necesita and for tú is necesitas. Both are given in the simple present tense.
