是
It is a waste of time to do something.
因为这是动词不定式的复合结构。
It's adj. for sb to do sth for sb to do sth是真正的主语，sb是to do sth的逻辑主语的。
如： It is impossible for him to get up early. It's important (for me) to carry on practicing speaking for learning English.
动词不定式的复合结构是“for / of sb. to do sth.”。
扩展资料
不定式复合结构的介词用for还是of,主要决定于前面形容词的性质。
一般说来,of前面的形容词是careful, clever, foolish, good, kind, nice, wise等,说明人的特性。
for前面的形容词是dangerous, difficult, easy, hard, heavy, important, interesting, necessary等,一般说明不定式动作的特性.
前面如果是名词用for。如:
1.It’s a good idea for parents to allow children to study in groups during the evening.
对父母来说,在晚上让孩子分组学习是个好主意。
2.It’s wise of him to do it well.
对他来说,把这件事做好很明智。
