Another kind of faulty sentence structure, called faulty predication, occurs when a subject and predicate do not fit together grammatically or simply do not make sense together. Many cases of faulty predication result from using forms of be when another verb would be stronger.

A person is not a characteristic.

Rules cannot expect anything.

Constructions using is when, is where, and the reason . . . is because are used frequently in informal contexts, but they may be inappropriate in formal academic writing because they use an adverb clause rather than a noun as their subject complement (see sentence grammar).