Hi Lamees, As it was already mentioned, both verbs express obligation. "Tener que" is often used when the obligation is imposed by strong external circumstances and the subject has no control over them. In English, it is expressed by "to have". Example. Tienes que poner el cinturón de seguridad si no quieres que te multen. (By law, it´s an obligation). "Deber" (in English must) is used to express a moral or internal obligation and has to do with what is desirable o accepted from the speaker's point of view. Example: Debo dejar de fumar (because I understand that is harmful to my health, not because it is a prohibition). Keep in mind that even native speakers use deber y tener often times interchangeably and that tener que is more frequent in spoken Spanish. I hope it helps to make it clearer to you!

DIFFERENCES BETWEEN TENER QUE + INFINITIVE AND DEBER + INFINITIVE. 1. TO EXPRESS OBLIGATION OR NEED. tener que + INFINITIVE is used to determine an external obligation that does not depend on the subject, but on the situation, on the circumstances and conditions. deber+ INFINITIVE is recognized as an obligation or need that requires the subject, expresses an opinion of what the other should do. Si quieres llegar a tiempo tienes que tomar un taxi. If you want to arrive on time you have to take a taxi. No debes decir esas cosas. Te pueden costar caras. You must not say those things. They can cost you faces. deber + INFINITIVE has less force of obligation than tener que + INFINITIVE. 2. TO EXPRESS PROBABILITY OR SPECULATION. tener que + INFINITIVE expresses something that can be very likely, almost certain. Different to personal opinion. deber (de) + INFINITIVE expresses a personal assumption, so the probability is less strong. -¿Qué hora es? What time is it? - No sé, pero deben de ser como las siete. I don't know, but they must be like seven. - Debe de estar en su casa, casi seguro. He must be at home, almost certainly. 3. tener que AND deber + INFINITIVE IN THE PAST. The difference in the simple past tense with these two verbs is that when we use tener que + INFINITIVE we talk about something that had to be done and in the end it was done, while with deber + INFINITIVE something in the end was not done. Tuvo que salir por se sentía mareada. She had to leave because she was dizzy. [He had to leave and there was]. Debió salir porque el juez pidió que abandonara la sala. He had to leave because the judge asked him to leave the courtroom. [But he didn't].

Él debe arreglar su cuarto ( should) Él tiene que arreglar su cuarto ( have to, must) Cuando dices "debe" puede ser solo un consejo , quizás una orden pero no tan estricta/ dura "Tiene que " es más una orden , si no lo haces va a haber consecuencias , Espero te ayude Saludos 😀

The answer Paola Cuervo gave is the best. Debo=something that you have the moral obligation to do, tengo que=when you have the precision to do something or take care about that.

It is very easy. Both have exactly the same meaning: strong obigation ( similar to must and have to in English) Mañana debo pagar la factura de la luz. Mañana tengo que pagar la factura de luz. The only difference is that we tend to use more "tener que" in spoken conversations.