Pragmatically, its use is constrained by economy of speech and understandability on the part of the hearer, hence its low frequency of actual use.

• 含该类主语的句子谓语部分是说话人对该主语所指事体做出的评论、描述或说明，其际运用受到话语经济性和接受者易解性等因素限制，使用频率较低。