1. 不是我不喜欢这辆车,而是它太贵了。 it's not that i don't like this bike, but it's too expensive
2. 不是我不愿意帮你,而是因为这段时间我工作太忙了。 it's not that i don't want to help you, but it's because i'm very busy during this period
3. 不是他不需要你帮忙,而是他怕给你带来麻烦。it's not that he don't need your help, but he dare to bring you troubles.