Abstract: Theories on the functions of copulas normally include Linker Hypothesis, Dummy Hypothesis and Predicator Hypothesis, all of which have their limitations, especially when applied to Chinese.In a cross-linguistic perspective, the present paper describes the copulas synchronically and diachronically, revealing that the functions of copulas are to indicate or emphasize the interpretation of “topic-comment”, i.e.predicate-indicator.Copulas are obligatory in more grammaticalized languages (such as English), and thus they are called grammatical copulas.In less grammaticalized languages (such as Chinese), the interpretation of “topic-comment” relation resorts to context and world knowledge.Copulas in these languages are optional and are thus called pragmatic copulas.