Los pronombres impersonales hacen la función del sujeto, el cual no se puede omitir en la gramática inglesa.
You
You learn a language by living in the country where it is spoken.
(Se aprende...)
It
It is ten o'clock.(Son...)
It's raining.(Está...)
It's cold.(Hace...)
It seems that....(Parece que...)
It was me.(Fue...)
They
They say that...(Dicen que...)
One
One likes to have a rest.(A uno...)