happen作“发生、碰巧”解,一般用于偶然或突发性事件
What happened to you?
occur作“发生、想到、突然想起”解,表示偶然发生时,其意义相当于happen
What has occurred?(=What has happened?)
take place 表示“发生、举行、举办”,一般指非偶然性事件的“发生”,即这种事件的发生一定有某种原因或事先的安排
Great changes have taken place in our hometown during the past ten years.