The verb to be can be an auxiliary (helping) verb or a linking verb. The linking verbs connect (link) the subject of a sentence with further information about the subject.
Examples of linking verb to be
My father is a surgeon.
The meeting was yesterday morning.
I have been back at home since last Monday.
Marion and Betty were really excited about their holiday.
They will be surprised by your present.
Compare: Helping verb to be
Read more: Verb to be forms and use
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Verb to be as a linking verb
Be links the subject with the rest of a sentence to give us further information about it. It is used to:
1. Indicate the existence of a person or thing.
Anchorage is in Alaska.
His portraits will be in the Gallery of Modern Arts.
There is a black cat on the tree.
There were three men in the backyard.
We can also say Three men were in the backyard, but There were three men in the backyard is much more common in English.
2. Describe the subject.
My grandpa was a genius.
You have always been my best friend.
Teak is a tropical hardwood tree.
3. Express a physical and mental state or a quality.
I have been ill for two weeks now.
You will be really tired after such a long walk.
Harold is stupid.
With some adjectives 'be' can be used in the continuous form, but it has a different meaning. Compare the following sentences.
You are foolish. (= You are a fool. It is your personal quality.)
You are being foolish. (= You are behaving foolishly now.)
Sarah is polite. (= Sarah is a polite girl. It is her personal quality.)
Sarah is being polite. (= Sarah is acting politely now, she is only pretending that she is polite.)
4. Express age.
How old are your brothers and sisters?
Tim and Bill are ten years old. They are twins. Sue is thirteen and Angie is sixteen.
5. Talk about sizes and weights.
Derek is 190 centimetres tall.
What is his weight? - He is 95 kilos.
What is the highest mountain in the Alps?
It is Mont Blanc, I think.
6. Talk about prices.
How much is this coat?
It is ninety-nine Euros.
How much are these chairs?
They are twenty dollars each.