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.    词根tempor表示time时间，因此该词基本意思是临时     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: ~ a barbeque pit at the campground.     improvise常表示艺术家即兴发挥，如演讲演奏演唱表演to improvise a jazz/solo/dance/opening speech/meal即兴爵士乐/独唱/舞蹈/开场演讲/一顿饭


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      即兴演讲用这个词，虽然extemporaneous和improvisedi也可用于讲话演讲，但强调临时、没有准备的发挥，正式的即兴演讲用本条impromptu


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临时: 强调出于临时需要，可能简陋粗糙，improvised临时即兴: 强调没有准备设备，impromptu即兴: 强调直接自然的即兴表达，offhand随便: 强调随意漫不经心，unpremeditated无预谋非故意: 表示由于冲动(突然激发的情绪)而做某事


记忆方法： 1）首字母ieiou想成5个元音字母aeiou<==因为即兴错把a写成i

          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.