The sentence is properly constructed (everyone is indeed singular), but the words uses are inaccurate as follows :
I endeavored on my final exam.
I have endeavored the darkest parts of the world.
Although it was already getting dark, he endeavored to complete his task.
When the supervisor shortened everyone's break by 10 minutes, he had a bunch of very disgruntled employees.
Going into high school is a huge milestone in my life.
The liberation of slaves was a milestone in the development of the country.
No this does not make any sense. What would be more proper to use is, "Will this opportunity be a milestone in your career?"
you are the new employee of the month!
I'm the best employee at Tescos
I fired an employee today.The employee tried his best to impress the manager.
"New" is the adjective in the sentence. It describes the noun "employee."
On the street everyone is in a rush