Mandarin Chinese has had a copula 是 shì for the last two millennia. It differs from verbs in not taking aspect clitics (-le pfv, -guò exp, -zhe ipfv). Its possible origin from a demonstrative pronoun is irrelevant for its more recent development since that demonstrative has long fallen out of use. In contemporary Mandarin, the copula is generally obligatory in clauses with a nominal predicate ().

In such a specificational clause, the subject is referential and the predicate designates a non-referential restricted set. In a, the predicate precedes the subject, with the effect of a pseudo-cleft construction.

In b, the order is again ‘subject - predicate’. With neutral stress, this has the first meaning indicated. With emphatic stress on the clause component following the copula, the effect is focusing on this component.

In most cases, the final nominalizer de is optional in this construction. Then shì is insertable at main constituent boundaries in a clause and separates its thematic part from its comment ().

The comment contains the focal component of the clause, including its verb. Shì is inserted as closely in front of the focal component as allowed by its positioning rules.

The syntactic structure of this focus construction is the same as in a copula clause. However, the meaning is not the specification of the concept designated by the component following the copula as being the referent designated by the component preceding it. Thus, b does not mean ‘I am the one who wants to buy that book tomorrow.’ Instead, the meaning is the application of the presupposed predication to the focal component, which is the one bearing emphatic stress – here, míngtiān. Two features of this focus construction are motivated by its function:

1. Use of the copula in this construction is motivated by this application relation.
2. The position of the copula in front of the focal component points towards the latter's focal function.

At the same time, feature #2 excludes an interpretion of the construction as an ordinary (specificational) copula clause since what precedes shì is not its subject. Here as in Jula, a could be an insistent cleft-sentence, while #b and #c must be monoclausal. Thus, shì develops into a focus marker.

References

Huang, Cheng-Teh James 1982, ‘Move wh in a language without wh movement’. The Linguistic Review 1: 369–416.

Prince, Kilu von 2012, ‘Predication and information structure in Mandarin Chinese’. J East Asian Linguist 2012, 38 pp.

Zhan, Fangqiong & Sun, Chaofen 2013, ‘A copula analysis of shì in the Chinese cleft construction’. Language and Linguistics 14(4): 755-789. [download]