# I have applied my heart

"I applied myself". See Ecclesiastes 1:17. 

# every kind of work that is done

"every kind of work that people do"

# under the sun

"on the earth". This refers to things that are done on earth. See Ecclesiastes 1:3.

# There is a time when a person exercises authority over another person to that person's hurt

Sometimes one person will oppress another and cause that second person to be hurt.