it is difficult to tell the differences.真正主语是后面的不定式to tell the differences
it is kind of you to help others.真正主语：to help others
it is said that the food is good for health.真正主语:that the food is good for health.
it is popular to keep fit by Yogu.真正主语：to keep fit by Yogu
这些都是it 为形式主语的句子.