I have seen so many of my students WERE forced to quit
pianos or other interests to be able to focus on .....
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see这个词看似简单，用法诡异。
描述已经发生或看到的事件，只有see sth或sb do/doing/done的句型，
不可采用宾语从句句型， 也就是说I saw that you stole his wallet
yesterday是错句，而
正确的
是I saw you steal （stealing）his
wallet yesterday.