"He is a boy."
Translation:Er ist ein Junge.
Einen is used for masculine nouns when they are in accusative case (ie, when they are the direct object of the sentence). In a sentence like "er ist ein Junge", both Junge and er are considered the subject (or nominative case) since they refer to the same person. It's like saying er = Junge.
So now I'm very confused about personal pronouns in German. As far as I know, "Ich" means "I" and "Du" means "You". And the rest of them is literally chaos. What do "er", "sie", "es" respectively mean? And What are "Wir", "Ihr" and "sie/Sie"? Also I wanna know the difference between "Sie" and "sie".
"Sie" is used when you address respectfully to someone and it can refer both to singular or plural form of "you". Wie heissen Sie ? - What is your name? (formal) - and think that this example stands both for singular and plural (addressing to a single person or many, but in a respectful way). "sie" = they. Nothing else special about it... Hope you understood something. I'm not good at explaining things. :D