Until last night, I didn’t have a clue. But thanks to The Copyeditor’s Handbook, by Amy Einsohn, I now know that copulative verbs are “verbs that express a state of being, rather than an action.” A few examples are be, become, feel, seem, smell, sound, taste.
Like good meditators, copulative verbs don’t just do something; they sit there and be. And because they express being rather than doing, they are unique among verbs in that they are modified by adjectives, not adverbs.
Here are some examples of modified copulative verbs, taken from The Copyeditor’s Handbook:
I am fine; he became sad; she feels bad; they felt ill; you seem happy.
This fish smells bad; the band’s new song sounds good; the souffle tastes delicious.
This next example, also from the Handbook, shows the dissonance that can result when an action verb winds up in the same sentence with copulative verbs:
Identical twins may look different, sound different, and walk differently.
See? Look and sound are copulative verbs, so they are modified by the adjective different. Then along comes walk, an action verb that requires the adverb differently. Jarring, isn’t it?
For euphony [harmony] change the final item to “have different ways of walking.”