Middle Verb


In English grammar, a main verb is any verb in a sentence that is not an auxiliary verb. Also known as a principal verb. A main verb (also known as a lexical verb or full verb) carries the meaning in a verb phrase. A main verb is sometimes preceded by one or more auxiliary verbs (also known as helping verbs).
Introduction to middle verbs
The middle verb is actually an archaic grammatical construct in English, but it is still useful to know what it means. Essentially, a middle verb is a verb that has been conjugated in such a way that the subject of a sentence acts upon itself. 
In some languages, for example, if you say that someone “hurt himself”, then hurt would be conjugated as a middle verb in order to express this action. 
In English, though, the reflexive pronoun (“himself”) is generally used in place of any actual conjugation of a middle verb. 
Understanding proper construction
Here is as good an example as possible of the middle verb being used correctly:
“When the man was shaving his face in the morning, he accidentally cut himself because he was distracted by thoughts of his upcoming meeting.” 
The reflexive construction “cut himself” is the closest that English comes to making use of the middle verb. 
Now, here is an example of the middle verb being used incorrectlyin a sentence:
“She had the cereal pour milk on itself in the morning, as she usually did in the morning when preparing breakfast.”
Unless something very surreal is meant here, “pour milk on itself” is an inappropriate reflexive and/or middle verb construction. 
Here are a couple rules, now, that you can follow in order to make sure that you are making use of the middle verb in a proper way. 
0. In English, there is no such thing as the middle verb per se. So in the proper sense, you cannot actually make use of it. The closest you will come to the middle verb is the reflexive construction, which indicates that a subject is performing an action on itself. These constructions are generally signified by words such as himself, herself, or itself. 
0. When you are using the reflexive construction in the place of the middle verb, the most important thing to keep in mind is that it is only used when a subject acts on itself. Or in other words, the subject is both the subject and the object of a given verb. The traditional middle verb was meant to be used only to convey this kind of meaning.  
Historical information
Historically, English did in fact have a middle verb construction in its linguistic past. However, over time, this construction has been completely phased out, to the point that it is no longer possible to use the middle verb construction within the context of modern English (since the construction simply doesn’t exist anymore). The middle verb thus provides a good case study of the way that language evolves over time, not only at the level of its vocabulary but even at the level of its basic syntax and structures. 
In other languages such as ancient Greek, the middle verb possessed its own voice and specific conjugations. That is, unlike in English, the middle verb in such languages possesses its own distinct conjugations, in a similar sense to how English verbs are conjugated differently depending on whether they are in the active voice or the passive voice. Again, though, the middle verb construction as a specific voice no longer exists in English. The middle verb’s function has been taken over by the reflexive construction.