I wish...would

在Cambridge文法書中講解 "I wish...would "的部分

常用在形容希望某是發生(但目前沒發生)

例如:

[Sarah現在不在,但我希望她出現]

I wish Sarah would come.(I want her to come)

I wish Sarah were here now.

但確不能用"I wish Sarah would be".

另外看到有個範例
"I wish the baby would stop crying"
此時卻可以用
"I wish the baby would be quiet"

我的問題是為何"I wish Sarah would be here"不能用?
但又為何能用"I wish the baby would be quiet"?