The verb "to be" is very often a linking verb. For example in the sentences "Bill is tall," "Shari was thin" and "He is my favorite singer," the verb does not express an action. "To be" is almost always either a linking verb or a helping verb (the difference is described in Section 4).
Some other verbs often serve as linking verbs. These include "feel," "look," "taste," "smell," "turn" and "become," as well as other verbs similar to these. These verbs sometimes act as action verbs and sometimes as linking verbs.
If the verb describes an action, then it is an action verb; if not, it may be a linking verb. For example, "I smell fish" describes an action I am taking, and here, "smell" is an action verb. But "I smell good" describes "me," and here, "smell" is a linking verb.
Some verbs, especially "to be" and "to have" sometimes function as helping verbs. If the verb is used with another verb, then it is a helping verb. For example, in the sentence "He was looking for a restaurant," "was" is a helping verb; it helps the verb "looking." But in the sentence "He had trouble," there is no other verb, and "had" is a linking verb.