sometimes在否定句中的用法：
副词的位置是be动词之后，也就是说在连系动词之后，行为动词之前，第一个助动词之后。在否定句中，则放在not之后。
sometimes变否定句时的位置，分情况，如下：
He sometimes goes skateboarding on weekends.(改否定句)
He never goes skateboarding on weekends。
Sometimes you're supposed to come early. (改为否定句)
Sometimes you aren't supposed to come early.
Jim is sometimes not very punctual. 吉姆有时不太准时。
He is sometimes not satisfied with my work. 他有时对我的工作不满意。
Debbie is sometimes not responsible for what she does. 戴比有时对她所做的事不负责任。