Here are some simple explanations:

Everyone means all the people
Everybody means all the people
Everyone and everybody are equal. They mean the same thing.

Teach means to provide new information to one or more people (the teacher teaches)
Learn means to gain new information or a new skill (the student learns)
Study means to read something to learn something. (the student studies)

Say and tell... see "https://www.italki.com/discussion/167466"