Good question. "lavarse" and "se lava" mean the same thing, but you have to use the lavarse construction when it immediately follows a preposition. So you could say either "(El) tiene que lavarse los manos" or "(El) necesita se lava los manos"
Another Example: Yo tengo que ducharme = I need to shower Debo me ducho = I must shower
Chris, you are partly right here. The verb form after a preposition must be the infinitive.
However, you've forgotten that the infinitive is used after a conjugated verb as well.
To correct your examples:
(Él) necesita lavarse las manos. He needs to wash his hands.
Debo ducharme. I must shower.
Are you saying that in Spanish the word "que" can be used as a preposition? Its translation, "that," is NOT an English preposition. Rather, "that" is an English demonstrative pronoun. Could you give an example of a sentence with a word that is a preposition in both Spanish and English, an example in which a reflexive construction like "lavarse" follows that preposition?
Branden, the construction tener + que + infinitive is used to say what you have to do. Your se lava has to be in the infinitive form.
Spanish generally doesn't have two conjugated verbs together, and your sentence would be translated something like [He has to he washes his hands].
This rule often works in English, too:
Tengo que lavarme la cara. I have to wash my face.
Quiero estudiar. I want to study.
And sometimes the infinitive in Spanish doesn't translate to an infinitive in English:
Puedo ayudar. I can help.
Debo comer. I should eat.
ChrisSerpe, reflexive verbs can be–and often are–translated as English present progressive tense. Thus, "Él tiene que se lava las manos" literally translates to "He must/has to is washing his hands."
Lavar = to wash (clothes, car)
Lavarse = to wash oneself (one's hands, one's face)
Lavarse is a reflexive verb. The se on the end of the infinitive indicates that, and when you conjugate it you use the reflexive pronoun before the verb:
Me lavo las manos. I wash my hands.
Te lavas la cara. You wash your face.
Se lava los pies. He washes his feet.
Nos lavamos el pelo. We wash our hair.
Se lavan los codos. They wash their elbows.
Lavo mi coche los sábados. I wash my car on Saturdays.
Me lavo el pelo todos los días. I wash my hair every day.
Much more at studyspanish.com (Grammar Unit Five).
Would it also be correct to translate: "He has to wash her hands"? If so wouldn't, "He has to wash the hands" be better? Now officially confused by two languages. Perhaps we need a new English term, "bi-bewildered" or "poly-perplexed".