To--- where For--- your intention, why ex: I'm going to the store for some groceries

it depends where you use them

"To" you to use for give direction, exemple I write one latter and gonna sand "to you" But if you gonna help you and gonna to write one latter for you cause you don't know to write so... You gonna write a letter " for you" ..... Sorry my english cause I speak Portuguese and I'm learn englesh

thanks guys

Here is a cool website that explains the differences rather well :) http://www.espressoenglish.net/difference-between-to-and-for/ Hope this helps :)