That seems to be a commandment of grammar that was drilled into people's minds by every well-meaning teacher.
The problem is that it's not entirely accurate. It would be more accurate to say, "Iscan be a linking verb." or "Isoften acts as a linking verb."
The truth is that is can also be a helping verb or an intransitive complete verb.
Henry is happy. (is = intransitive linking verb)
Henry is walking. (is = helping verb)
Henry is here. (is = intransitive complete verb)
We'll go over each of those sentences, and I'll show you how they work.
Note that this lesson applies to other forms of to be (am, are, was, were) as well.
When is is a linking verb
Henry is happy.
The job of a linking verb is to link the subject with either a noun that renames it (predicate noun) or an adjective that describes it (predicate adjective). In this example, is is linking Henry with the adjective happy. It's saying Henry = happy.
Because it's clear that the predicate adjective happy refers to the subject, we know that is in this sentence is linking.
More linking verb examples
Henry is the president of the school's science club. (Henry = president)
My younger sister is my best friend. (sister = friend)
This fajita is amazing! (fajita = amazing)
When is is a helping verb
Henry is walking.
Helping verbs are verbs that "help" the main verb in the sentence. That means that they don't carry the weight of the meaning of the verb. In this case, walking is the main verb and is is a helping verb.
More helping verb examples
We are learning about grammar. (are = helping verb, learning = main verb)
Our car is running. (is = helping verb, running = main verb)
The kids are biking to the store. (are = helping verb, biking = main verb)
When is is an intransitive complete action verb
Henry is here.
This is the one that trips people up. There are three types of action verbs (you learn about all of them in our Get Smart program), and in this case, is functions as an intransitive complete action verb.
This means that the verb doesn't transfer its action to anyone or anything. It is complete all by itself.
If that's hard to wrap your head around, a synonym for is in this case is exists. (Henry exists here.)
More intransitive complete action verb examples
The cookie dough is in the back of the fridge. (is = intransitive complete verb)
Jeremy and Martha will be here soon. (be = intransitive complete verb)
My friend's cat is in the tree. (is = intransitive complete verb)
Hello! I'm Elizabeth O'Brien, and my goal is to get you jazzed about grammar.
you! Thank you! Thank you! I'm 47 years old and have begun to
understand grammar for the first time! I have a college degree and feel
more comfortable helping my four girls with school than ever before.
- Christine, Parent
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