I don't know what I can do for you 为什么用what不用that

I don't know what I can do for you 为什么用what不用that
I don't know what I can do for you 为什么用what不用that

I don't know what I can do for you 为什么用what不用that
what引导的宾语从句,what可以在从句中作主语或宾语,在此句中就充当宾语.I can do what for you
而that引导名词性从句时没有实际含义,也不充当句子成分

缺宾语用what，而that指代同位语或是、、、

I don't know what I can do for you 意思是：我不知道能为你做些什么
I don't know that I can do for you ：我不知道我可以为你做事
I don't know（ that I can do for you）从句

i don't know what i can do for you 意思是，我不知道能为你做些什么
i don't know that i can do for you 意思是，我不知道我能为你做