A. The verb subject
The personal pronoun (I, you, etc) is used for emphasis. It is not part of the conjugation. For example, when you want to say “I am working today”, you don’t need it. However, if you want to say “Me, I am working today”, you must mention it.
“Είμαστε” → “We are” (without the subject)
“Εμείς είμαστε” → “It’s us.” (with the subject)
The 3rd person can be masculine, femine or neutral, but the verb ending is the same for all three.
Ο Γιάννης είναι στο σπίτι. (Yannis is at home.) (masc.)
Η Μαρία είναι καλά. (Maria is fine.) (fem.)
Το ταξί είναι γεμάτο. (The taxi is full.) (neut.)
“Εσύ” is used when talking in a friendly, infomral way (family, friends, children to each other)
“Εσείς” can either be used as an informal “you” for 2 or more people or as a formal “you” when talking to 1 person with due respect (the elderly, an older person, your boss, strangers in general) or coldness.