Keep our old friends is more important than make new friends.此句错误.原因正如楼主所言,相关语法点规定如下：动词原形构成的短语是不能直接充当句子的主语或表语的.需要改成动名词短语或不定式短语,而且前后的...那I understand that communication with others and expression ourselvesis very important也是不对的吧？应该写成：I understand that communicating with others and expressing ourselvesis very important.对么？没错。第一个主语用communication/communicating都可以。但是第二个主语用expression ourseves就不对了，只能说成expressing ourselves。