fire
"Is" is the verb in that sentence.
The verb is: is
They fire the cannon every day at midday.
"Fuelling" (by which I assume you mean "fueling") may be used as a noun (gerund) or verb: Fueling the car was expensive. (Noun use) The log was fueling the fire. (Verb use)
No, "Her brother's car" is not a sentence, it is a noun phrase; it has no verb. For example:Her brother's car is new. (the noun phrase is the subject of the verb 'is')She's driving her brother's car. (the noun phrase is the direct object of the verb 'driving')
"Could have forgotten" is the complete verb phrase in the sentence, "Could you have forgotten your sunglasses in the car."
His boss decided to fire him because he was the one that had started the fire in the warehouse.
It has a subject (It) and a verb (is), and it makes sense. Therefore it's a sentence.
"is" is a verb.
should yield.
We exited the building via a fire door.
is broken