I can think of many cases students obviously knew a lot of English words and expressions but couldn' t write a good essay.
Who is it up ____ decide whether to go or not?
Wang Mei is one of many Chinese teenagers _____ “on the go”
He was born ____ a very cold night _____ December.
When he knocked at the door, my mother was busy ____ supper.
He waited until it was dark ____ nobody could see him clearly.