to think very hard about something
pensar muy duro sobre algo (pensar muy duro sobre algo)
"I had to rack my brains to remember her name."
"Tuve que rack my brains para recordar su nombre." (Tuve que **rack my brains** para recordar su nombre.)
to do something revolutionary or extremely difficult
to suddenly start crying
To spend all your time studying
to commit an illegal act
To challenge someone to fight or compete.
to fulfill your part of an agreement or duty