下面的内容摘自《葛传椝英语惯用法词典》:
下面每句里的 that 可有可无:
I would do it with pleasure, only (that) I am too busy.
He would have come yesterday, only (that) pressure of
work did not allow him to come.
用了 that 意思重在那主要的从句,不用
that 意思重在那附属的从句,如:
I would do it with pleasure, only that I am too busy. 假使我不太忙,我将乐于做这个。
I would do it with pleasure, only I am too busy. 我将乐于做这个,可是我太忙了。
下面两句里不可加上 that:
He is very careful, only he sometimes makes mistakes. 他很小心,可是他偶尔还会犯错误。
You may come at any time, only you had better let me know
beforehand. 你可以随时来,可是你最好预先通知我。
为什么前面两句的 that 可以有可无(只是侧重点有所不同),而后面两句却不可以加上 that?