A copula is a word used to link the subject of a sentence with a subject complement, often describing or identifying the subject. The most common copula in English is the verb "to be," which can take various forms such as "is," "are," "was," and "were." For example, in the sentence "The sky is blue," "is" serves as the copula connecting "the sky" to its description "blue." In linguistics, copulas can also refer to other verbs that serve a similar function, such as "seem" or "become." Different languages may have unique copula forms or may not use a copula at all in certain contexts. For instance, in Russian, the copula is often omitted in the present tense, as in "Он студент" (He [is] a student).