spend time on doing sth.或者spend time doing sth.on是可以省略的
spend sth on sth也是可以的
speaking practice doing sth.(练习做某事),practice后面的动词必须加-ing,固定用法.问题补...而practice无论本身是什么形式,后面的动词都要加-ing,明白了吗?其用法与"enjoy,finish"等类似.