The pen that (which)he lost yesterday has been found .
他昨天丢失的钢笔现在已经找到了。
The pen that (which)was lost yesterday has been found .
昨天（被）丢失的钢笔现在已经找到了。
The book, which he lost yesterday, has been found.
这就是他昨天丢失的那本书。
The dog which was lost has been found.
走失的那条狗已经被找到了。
The dog which was lost has been found.
走失的那条狗已经被找到了。
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