Expressing an obligation: 1) Hay que + infinitive is used for expressing the meaning of “it is necessary to do something” or “one must do something.” It is a general expression, without a subject, so the verb form hay is always used. 2) Tener que + infinitive is used for expressing obligation or necessity. It can be translated as “someone has to do something.” The verb tener is changed according to the subject of the sentence.