The simplest verb phrase (VP) consists of just a head verb, which is always a lexical verb. So, in the utterance the boy hugged the dog, the verb phrase is simply the head lexical verb hugged. This can be represented diagrammatically by using what is known as a tree diagram.
The head lexical verb in a verb phrase may, however, be modified by the addition of auxiliary verbs.
Recall that the primary auxiliary verbs are be, have and do. Auxiliary verbs in verb phrases always precede the head lexical verb, i.e. they are pre-modifiers. So, for example, our utterance may be modified through the use of the auxiliary be, as represented by the following tree diagram.
Similarly, it may be modified through the use of the auxiliary have as shown below.
In addition to the primary auxiliary verbs, recall that there is also a small set of modal auxiliaries. These indicate the mood or attitude of the speaker and include items such as can, could, should, must and may. Again, returning to our main example, this could be modified as follows.
It is, of course, also possible that a head lexical verb may be modified by both a modal and primary auxiliary verb, e.g.