Copular and linking verbs (be, become, seem, appear, feel, smell, taste, sound) don't describe actions but rather states or qualities. They connect the subject to a subject complement that describes or renames it. In "She is a teacher," the linking verb "is" connects the subject to the complement "teacher" that identifies her.
Identify linking verbs by testing if they can be replaced with "is" or "am" while maintaining meaning. Practice distinguishing linking verbs from action verbs by noticing that linking verbs describe what the subject is, not what it does.
From your study of verbs, you know that verbs are the engine of a sentence — they express what happens. Most verbs express actions: run, build, think, destroy. But a special class of verbs does something fundamentally different: instead of expressing an action, they express a state of being or a condition. These are copular verbs (from the Latin for "link"), and the most common of them is simply *be* in all its forms — is, are, was, were, will be.
The grammatical job of a linking verb is to connect the subject of the sentence to a subject complement — a word or phrase that describes or renames the subject. In "The soup *is* hot," the verb *is* does not describe what the soup is doing; it tells us what the soup *is*. The adjective *hot* is the complement, attributed back to *soup* via the linking verb. This is the core test: can you replace the verb with a form of *be* and have the sentence still make sense? "The soup smells good" → "The soup is good" — yes, that works, so *smells* is functioning as a linking verb here. But "The dog smells the food" → "The dog is the food" — that fails, so *smells* is an action verb there.
The most common linking verbs beyond *be* fall into two groups. Verbs of the senses — *look, sound, feel, smell, taste* — can function as either action verbs or linking verbs depending on sentence structure. The trick is whether the word after the verb is an adjective (complement, linking use) or a noun/adverb phrase (action use). "She looked tired" uses *looked* as a linking verb with the adjective *tired*. "She looked at me" uses *looked* as an action verb. Verbs of becoming or remaining — *become, grow, turn, remain, stay, seem, appear, prove* — always link the subject to a description of what it has come to be or continues to be. "The leaves turned golden." "The situation remained tense." In each case, the word after the verb answers the question "What is the subject like?" not "What is the subject doing?"
The most important grammatical consequence of linking verbs is that they require an adjective as complement, not an adverb. This trips up many writers. You should say "I feel *bad*" (adjective, describing your condition) not "I feel *badly*" (adverb, which would absurdly mean your sense of touch is impaired). The same logic applies: "The music sounds *beautiful*" not "sounds *beautifully*." Adverbs modify verbs; adjectives modify nouns. Because linking verbs attribute a quality to the subject (a noun), the complement must be an adjective. Getting this right requires correctly identifying whether the verb is linking or active — which is exactly what the substitution test gives you.