She can dance beautifully.為什麼不等於 She can beautifully dance.?

dance是「不及物動詞」，修飾它的情狀副詞只能放在它的「後面」：
She can dance beautifully.

類似的例句：
You work hard.
She sings well.
He drives carefully.
They laughed happily.

「及物動詞」則修飾它的情狀副詞可放在動詞前面或受詞後面：
He picked up the ball quickly.
= He quickly picked up the ball.
但 well, hard, badly等情狀副詞則只能放在受詞之後。
He speaks English well.不能寫作He speaks well English.

修飾形容詞或副詞的情狀副詞通常放在前面：
He is dangerously ill.
It flies wonderfully fast.

修飾全句的情狀副詞放在句首：
Happily he did not die.（幸而他未死；副詞修飾全句）

He did not die happily. （他死得不幸；副詞只修飾動詞）
請參見柯旗化《新英文法》pp.483~486

She can dance beautifully.

*很簡單: S➕助動詞(必須跟root verb原動詞）➕副詞

*「beautifully」就是形容「dance」的副詞, 一般放在動詞後面。

另例：Birds can fly high.

beautifully 為情狀副詞,一般來說是放在動詞後面,但如動詞後有受詞,則放在受詞之後
Tom runs slowly. -- slowly為情狀副詞
I like to listen to music quietly. --quietly為情狀副詞

前面那句是對的但後面那句是錯的