剛google 了一下，劉全國的解釋比較全

作主語：Seeing is believing.百聞不如一見。Talking is easier than doing. –ing作主語時，如果其結構較長，可用it作形式主語，而將作主語的-ing後置。如：It isn’t much good writing to them again. It’s no use waiting here.
-ing形式的複合結構。在-ing前加物主代詞或名詞所有格即構成-ing的複合結構。其中的物主代詞或名詞所有格為-ing的邏輯主語。這種結構在句中可作主語、賓語或表語，如：Your smoking and drinking too much will do harm to your health. 但在口語中，這種結構如作賓語用，其中的物主代詞常用人稱代詞的賓格，名詞的所有格常用名詞的普通格代替，如：She insisted on Peter’s （or Peter）going there first.

所有跟帖： 

• you did not understand grep's question -justjust- ♂ (115 bytes) () 05/08/2009 postreply 20:09:55

• likely people would put -justjust- ♂ (33 bytes) () 05/08/2009 postreply 20:20:51

• I agree with you, just. this form can be a subject. -戲雨飛鷹- ♀ (0 bytes) () 05/08/2009 postreply 21:32:37

• I might have neglected that ... -billnet- ♂ (756 bytes) () 05/08/2009 postreply 21:57:21