A middle verb is a word that has elements of both active and passive voice, indicating action taken by the subject of a sentence that is not performed upon an object. Active voice occurs when the subject of the sentence performing an action comes first, while passive is formed when the subject comes after the action. A middle verb has the structure of active voice, with the subject coming before it, but the information is presented in a passive way. For example, a statement like "The car starts quickly," has the verb "starts" which refers to the subject "car" but is also performed by an unnamed secondary subject.
Several basic elements of a word make it a middle verb, beginning with the idea that it describes an action, like other types of verbs. There are not necessarily specific words that are "middle," but many different verbs can take on this form within the proper context. The word "starts" in the previous example can easily act as a regular, active verb in a sentence like "The man starts the car," which is similar in structure to the example. It is not a middle verb in this second use, however, as the subject is performing the action upon an object.
Additionally, a middle verb is also intransitive, which means that it does not require an object to make sense. In the second example, "The man starts the car," the word "starts" is transitive because it requires the object "car" to have any meaning. "The man starts" does not provide enough information for someone to be able to understand what the man is doing in that example. In the sentence, "The car starts easily," however, it is a middle verb, because there is no object in this sentence and yet it still makes sense.
When a verb is used in active voice, and it is intransitive, then it may be a middle verb, depending on how the sentence is structured. "The cat slept," is not an example of this, because it is only in active voice. For a middle verb to be used, there is an implied passiveness to the sentence.
In the example of "The car starts easily," there is a secondary element to the sentence, which is the fact that someone else is starting the car. To be truly in active voice, it should be something like "The man easily starts the car," but in this sentence "the car" is the object and not the subject. The meaning is the same between these two examples, which is why the previous one has an element of passive voice along with the primarily active structure.