In terms of their distribution, verbs are words that can appear after auxiliaries. In the frame sentence (5), repeated for convenience, can is the auxiliary:

(5) She can ____.

We will have more to say about auxiliaries later. For now, we can simply note that they are words like can, could, will, would, shall, should, may, might, or must. In most grammar books, auxiliaries are considered a special type of verb, and that is how we will treat them. But it's important to note that auxiliaries do not behave like most other verbs. In particular, they fail most of the tests for verb-hood given here. For example, the frame sentence (5) cannot be filled in with another auxiliary.

(12) *She can might.[1]

That fact has led some linguists to treat auxiliaries as a separate word category. We will explore the logic for keeping them as a subclass of verbs when we examine the structure of verb phrases.

Morphologically, verbs change form to distinguish tense, and, in the present tense, the third-person-singular from other persons and numbers. Thus we contrast They walk, the present tense, from They walked, the preterite (past tense), and He/she/it walks from I/you/we/they walk. Verbs also take the suffix –ing.

Note, however, that these morphological tests don't work for every verb. Just as there are some exceptions as to how nouns form the plural, there are some exceptions to how verbs form the preterite. Almost every verb does allow -ing to be added, but there are one or two odd cases, such as beware.

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Notes

[1] In some regional varieties, of English, for example in North Carolina, two auxiliary verbs actually can appear together in the so-called double-modal construction, e.g., "I might could loan you the money." Such sentences, though, are ungrammatical for all the standard varieties of English.