After impersonal expressions that indicate a willing, wish, command, preference, desire, necessity, opinion, etc. on the part of the speaker that another person do (or not do) something, the subjunctive is used in the dependent clause, after the word que:
 

Es importante que usted hable con la directora.	It’s important that you speak with the director.
Es necesario que Elena tome un taxi.	It’s necessary that Elena take a taxi.
¡Es terrible que no sepas hablar español!	It’s terrible that you don’t know how to speak Spanish!
Es posible que sea el efecto de la comida.	It’s possible that it is the effect of the food.
No es preciso que usted tome medicina especial.	It’s not necessary for you to take special medicine.

However, if the expression does not reflect the desire to influence a specific person to do something, but instead is a statement that people in general should do something, then the infinitive (-ar, -er, -ir forms), and not the subjunctive, is used.
 

Es necesario evitar el alcohol.	It’s necessary to avoid alchohol.
Es preferible ir en autobús.	It’s preferable to go by bus.
Es posible comprar comida aquí.	It’s possible to buy food here.