What term is used to describe the verbs that express mood or tense?

The term that describes verbs used to express mood or tense is "modals." Modal verbs, such as can, could, may, might, will, would, shall, and should, are integral in altering the mood of a sentence, indicating possibility, necessity, ability, or permission. They help convey the speaker's attitude toward the action or state described by the main verb.

In contrast, while auxiliary verbs (often termed helping verbs) work specifically with main verbs to form different tenses, aspects, or voices, such as "is walking" or "has eaten," they don't encompass the full range of modality that modals provide. Action verbs are those that denote specific actions and do not inherently express mood or tense on their own. Linking verbs serve to connect the subject of a sentence with a subject complement, rather than expressing mood or tense. Therefore, "modals" is the most appropriate term when discussing verbs that convey such nuances.