Identify the error in this sentence: "The physician started to perform the operation, but the patient was not yet completely under anesthesia."

The phrase in the sentence that contains an error is "started to." In standard English grammar, the verb "start" should typically be followed by the gerund form of the verb that follows it instead of the infinitive form (which includes "to"). Therefore, the phrase should be "started performing the operation" instead of "started to perform the operation."

Using the gerund maintains the correct structure for this type of expression, which describes an action that is beginning. In this context, shifting from the infinitive "to perform" to the gerund "performing" aligns better with common English usage for indicating the start of an action.