extemporaneous: stresses the demands imposed by the occasion or situation and may imply a certain sketchiness or roughness: an ~ shelter prompted by the sudden storm.         sketchy梗概大略不完备的rough粗糙


improvised: implies the constructing or devising of something without advance knowledge, thought, or preparation and often without the necessary or proper equipment: ~d a barbeque pit at the campground.


impromptu: stresses the immediacy and the spontaneity of the thing composed or devised: an ~ speech at an awards ceremony.       immediacy直接，这里表示没有准备  spontaneous自发自然流露


offhand: strongly implies casualness, carelessness, or indifference: his ~ remarks often got him into trouble.


unpremeditated: suggests some strong, often suddenly provoked emotion that impels one to action: ~ murder.


extemporanesous，improvised，impromptu即兴,offhand随手而来: 随意不关心，unpremeditated无预谋非故意: 突然激发的


记忆方法： 1）ieiou和5个原因字母aeiou很像。

           2）即兴的意思是临时而为未预先准备mean composed, done, or devised at the moment and not beforehand.