Copula

A copula connecting word, in particular a form of the verb be connecting a subject and complement. 

Copula - An unusual verb tense

Although it sounds like the name of some film director, the copula is actually a part of grammar. A copula is a word (usually a type of verb tense) that links the subject and predicate of a given sentence. The verb that serves a copula does not exactly function as a verb. This is because the copula does not so much describe an action as a state of being.  

Used to link subject and predicate

Here is an example of the correct use of a copula within the context of a sentence. 

"Mary was tall for a woman, especially when one considered the fact that both her parents were relatively on the short side."

In this sentence, "was" is a copula that is being used correctly, as is "were". 

Now, here is an example of the copula being used in an incorrect way. 

"The sky was being a reddish color due to the presence of some kind of weird atmospheric effect that no one could explain."

In this sentence, the verb "being" should not follow the copula "was"; this is an awkward grammatical construction. 

Here a couple rules you can follow when using the copula, in case you are still a little confused about this part of grammar. 

1. In English, the most common copula is some form of the verb "is". This is because this verb can easily express a state or aspect of being: for example, it can say that the sky is blue, or that the woman is tall. This is almost always how the copula is used in the English language; and as such, it is common to find it in what are actually quite simple sentences. 
2. The copula may be more complex in other, non-English languages. For example, some languages make use of a different copula depending on whether the aspect of a thing being described is a permanent aspect or a transient aspect. Such distinctions, however, are not present in the English language. 

The purpose of copula in language

The primary purpose of the copula is a descriptive one: it is used to express a relationship between a given object and the properties possessed by that object. The copula can thus be understood as fulfilling one of the most basic functions of language: it allows people to describe their worlds and experiences in a direct way. For example, by using the copula, one can say that the animal was big, or that the weather was stormy. The copula is thus most common when the entire point of a sentence is to provide this kind of description of objects.

On the other hand, the use of the copula is perhaps somewhat less common when sentences grow more complex. For example, the copula in the sentence, "The sky is blue" could be eliminated by instead using the phrase "the blue sky". This would then enable the writer to express a more complex thought, instead of simply focusing on the basic fact of the color of the sky. Again, though, if the entire point of the sentence were to call attention to the color of the sky, then it would in fact be appropriate to keep the sentence simple and make use of the copula.