Answer:
In Spanish, verbs in plural end in "-an" or "-en" and when they are in singular they end in "-a" or "-e". The context is taken from the complement of the sentence.
Explanation:
Se comen comida sana en este restaurante is incorrect because comida sana, is singular, therefore the correct sentence is: Se come comida sana en este restaurante.
Se prohiben fumar en la sala de espera is incorrect because fumar en la sala is singular, therefore the correct sentence is: Se prohibe fumar en la sala de espera.
Se necesitan ejercicio para estar saludable is incorrect because ejercicio is singular, therefore the correct sentence is: Se necesita ejercicio para estar saludable.
Se necesitan operaciones de rodilla is correct, as operaciones is plural.
Se necesitan más doctores en este hospital is correct, as doctores is plural.
