what are you doing today?...

between use only two person, things ,place and among More than two ......

my mother is housewife....

pronoun use in place of noun ...for exmpl...1 ram is a student, he(pronoun) is going to school.

...on the earth

i am from Ludhiana

it's singular form.. piece of information, piece of bread ,piece of paper ......I think there are after Do not use " s / es "

i think...I study in 4th...( standard/ class...)

my rank in city

not use before name( itself ) Article ...A,An,The