The words that can appear in the blanks are neither main verbs nor adjectives, but rather words
like will, can, shall and must. In English, there is clear evidence that these verbs are different
from main verbs, and we call them auxiliary verbs (Aux). The auxiliary verb appears in front
of the main verb, which is typically in its citation form, which we call the ‘base’ form. Note the
change in the main verb form in (19b) when the negation is added: