哈哈,朋友.
对的,这些是老外的固定用法,要多记多背.
1.sth needs to be done = sth needs doing.这里的need做实意动词.意思是某物需要被怎么样了.一般这种搭配的主语是物品.
2.而I need to do sth.一般是人来主动做的.也为实意动词.
3.当然need也可以作为情态动词,但主语必须是人.Sb need do sth.第三人称单数记住不要加s.例如If she wants sth,she need only ask.
4.enjoy 加doing 固定搭配
5.mind 的正常展开式是Would you mind (sb's) doing sth?或Would you mind (sb的宾格：me,you,him) doing sth?
6.I am on a diet.老外一般是这么说的.I am dieting也对.
7.I am on vacation.－>对.但是vocation 只能作名词.
至于vocationing的确是一个新词,潮流词,但还没入正统,建议写作不要用.有个老外开了加公司叫VocationVacations,于是他做了这个词vocationing