(Appease: to pacify or quiet, esp. by giving in to demands) The overwhelmed mother was able to appease her whining children with the promise of an ice cream cone on their way home from the grocery store. It is nearly impossible to appease someone who is a stickler about grammar.
So- you are asking when to use 'when' in a sentence. When you are asking how to use when in a question, you are already using when in a sentence, because a question actually is a sentence. I like to use when in a sentence whenever I like.
To placate is to calm, pacify, or mollify. Example : "The airline gave out free drinks in an effort to placate angry travelers." Example : "In between, I placate her with more cake." (Sarah Miller in Miss Spitfire: Reaching Helen Keller.)