whenever意为“无论何时”，相当于no matter when; when ever意为“究竟何时”，相当于when on earth。例如：
Come to see us whenever you can. 你什么时候有空，就过来看看我们。
Whenever we met with difficulties, they came to help us. 每当我们遇到困难时，他们总是来帮助我们。
When ever did you leave the classroom last night? 昨晚你究竟什么时候离开教室的？