你是说没有明确的逻辑主语的吗?那好像不是定语从句把,是名词性从句中缺主语还差不多……
如：We can easily get from the computer__what__ has been strored in it.（__what__ has been strored做get的宾语,是主语从句）
或者：when I try to find what it is that prevents so many people from taking part in the program,it seems to me that there are two causes
中的that从句不是名词性从句,而是强调句式“it is that……”