This question already has an answer here:
I cannot find the answer to this in the EL&U archives but I am sure it must be there somewhere and that this is a duplicate question.
He/she goes to the shops every day.
We/they go to the shops every day.
Why am I plural ?
I go to the shops every day.
EDIT : The supposed duplicate is an interesting question and has a very competent answer but the answer is very broad and does not specifically answer my own question. People learning English often err in this very matter and it would be interesting if someone could answer more specifically to give an orderly reason why we do something in English that appears to be either completely chaotic or strangely contradictory.