修改病句：On Betty's desk has a chair.

是个倒装句,应该为
On Betty's desk is a chair.
意思是,Betty的桌子上有一把椅子.
如果是桌子旁边的话,把On改为Beside就行了.
另外,用there be句型的话,参考如下：
There is a chair beside Betty's desk.
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