The operation是主语,谓语部分是proved to be very difficult ,which lasted for over four hours为非限定性定语从句,to在这里是不定式的符号,短语prove to do something.BE +形容词表示性质、状态.之所以用covering不用covered是因为这里应该使用现在分词做后置定语,表主动含义,而用过去分词做定语表被动含义.It was difficult because of the hard resin covering the skin.=It was difficult because of the hard resin that/which covered the skin.（持续4个多小时的手术很难是因为覆盖在皮肤上的树脂）注意是说 the hard resin covered the skin.It was difficult because of the hard resin covered the skin.=It was difficult because of the hard resin that was covered the skin.注意the hard resin was covered the skin.这个句子是不成立的,应该说the hard resin was covered by/with the skin.但是句意又是不正确的.整句的意思成了手术很难是因为树脂被皮肤覆盖. The operation是主语,谓语部分是proved to be very difficult ,which lasted for over four hours为非限定性定语从句,to在这里是不定式的符号,短语prove to do something.BE +形容词表示性质、状态.之所以用covering不用covered是因为这里应该使用现在分词做后置定语,表主动含义,而用过去分词做定语表被动含义.It was difficult because of the hard resin covering the skin.=It was difficult because of the hard resin that/which covered the skin.（持续4个多小时的手术很难是因为覆盖在皮肤上的树脂）注意是说 the hard resin covered the skin.It was difficult because of the hard resin covered the skin.=It was difficult because of the hard resin that was covered the skin.注意the hard resin was covered the skin.这个句子是不成立的,应该说the hard resin was covered by/with the skin.但是句意又是不正确的.整句的意思成了手术很难是因为树脂被皮肤覆盖.