“是”除了用作繫詞，以表示等同／類屬關係外，還往往在另有表意動詞的句子中出現。一般以為在此類句子中，“是”的功能在於“聚焦”或“強調”。本文首先考查“是”的使用條件，指出“聚焦”說及“強調”說皆未能妥善解釋其使用上的限制。最後提出“是”的功能應歸結為一種人際層面上的特別表述（marked assertion），而此功能本為其原來繫詞功能的自然延伸。

Other than its typical copulative function, the verb “shi” often appears in sentences along with other semantic verbs. A common view goes that “shi” in these sentences carries a “focusing” or “emphatic” function. However, investigation on the use of “shi” demonstrates that such a view would fail to account for the restriction of its pragmatic distribution. It is argued that the function of “shi” is a kind of “marked assertion” in the interpersonal dimension. And this function is derived from its copulative function.