There are many children dropped out of school 這句中的dropped爲什麼

題目：

There are many children dropped out of school 這句中的dropped爲什麼改成dropping
老師說不能有兩個謂語,但過去分詞dropped做非謂語表被動不可以麼?
還有就是 there be 可以+n+V的那些形式?

解答：

drop和children之間是主動關係,dropping表主動,dropped表被動,所以選dropping