He was heard singing when .不还原to 吗？什么情况要还原to?原句就是ing 就是正在唱歌，转换为被动也要保留这种状态。有一些使依动词变被动时候加to 比如make sb. do 换成sb be made to do.. 还有 一些固定的复合谓语，比如he was reported/said to....