"Having conversations with friends was not helpful at all"中主语是什么,改为

主语是“ Having conversations with friends ” 这句话是动名词短语作主语
动名词短语,动词不定式都是非谓语动词的一种,都能作主语,只不过形式不同.
所以可以有如下同义句 ：
1.It was not helpful at all to have coversations with friends.
（“It”作形式主语,真正的主语是后面的不定式——“to have coversations with friends”,这一种是最常用的表达方式.因为句首用形式主语可以避免“头重脚轻”的后果.）
2.To have coversations with friends was not helpful at all.
（这句话中,“To have coversations with friends ”是真正的主语.虽然从语法上说没有错误,但是在表达上明显不如1句那么纯正、地道.）