Which word is used incorrectly in: "The man's heart attack scared him badly and he decided having a regular check-up from then on."?

The choice that is used incorrectly in the sentence is "having." The phrase "decided having" is not grammatically correct because it implies a continuous action, which doesn't fit well with the verb "decided." Instead, the sentence should use a structure that clarifies the action of deciding to have something in the future. A more appropriate formulation would be "decided to have" a regular check-up. This structure effectively conveys the intent to take action after the event of the heart attack.

Using "decided to have" not only enhances clarity but also adheres to standard English grammatical conventions. The use of "to" here transforms the action into an infinitive form, indicating a future intention.