We have seen young children piaying together.变为被动语态

Young children have been seen playing together by us.
如果您认可我的答案,请点击下面的“选为满意回答”按钮,为什么呢，帮忙解释一下原句的主语变成宾格，再加上by，就变成了宾语。放在句子的末尾。那为什么用have beenseen 呢，而不是were seen主动句变被动句，时态不用变。原句是have seen 为什么改后是have been seen因为have seen是主动语态，与之对应的的被动形式是have been seen现在完成时里，主动变被动都这么变，中间加个been就行了 如果您认可我的答案，请点击下面的“选为满意回答”按钮，谢谢！