According to the Principle of Topic and End-focus, as we have noted earlier, a sentence begins with reference to ‘given’ information and proceeds to provide ‘new’ information. There are many occasions, however, when we must construct sentences whose content does not fall neatly into the two categories of topic focus.
Yī gè rén lái zhǎo nǐ.
(Somebody came to look for you.)
Liǎng gè xīn lǎoshī lái dào wǒmen xì.
(Two new teachers have come to our department.)
Jǐ gè xuésheng xuéxí shàng yǒu kùnnan.
(A few students have troubles in their studies.)
All the subjects are indefinite, thus expressing new information. The hearer will experience a sense of awkwardness if he is expected to interpret a topic as entirely new and unconnected with anything previously introduced.
In such circumstances, some grammatical devices are invoked to postpone the item of new information to a more focused position. One of them is the dummy 有‘yǒu’ (there be) sentence. The effect of the prelude 有‘yǒu’ is to enable the speaker to place an indefinite NP in a position near the end of the sentence, thus avoiding its occurrence in the unfocused position of subject. This gives rise to the Chinese dummy 有‘yǒu’ structure.
Yǒu rén lái zhǎo nǐ.
(There is someone who came to look for you.)
Yǒu zhī niǎo fēi dào shùshàng le.
(There is a bird flying over to the tree.)
Yǒu jǐ gè xuésheng xuéxí shàng yǒu kùnnan.
(There are some students who have difficulties in their studies.)
To a native speaker, they all sound natural as they all begin with a dummy 有‘yǒu’, which, being a conventionalized linguistic means to call the hearer’s attention to what is to come up next, carries lowest CD, while the indefinite NP expressing new information is postponed to the end of the sentence. Such a device makes the sentences progress in agreement with the Topic and End-Focus
Principle. And as they all introduce a new entity and a related proposition (e.g.,in (2b) the related proposition is that ‘a bird flying onto the tree’ into the discourse, they are called existential 有‘yǒu’ sentences.
 Loar, J. K. (2011). Chinese syntactic grammar: functional and conceptual principles.
New York: Peter Lang.