These three verbs carry ability, obligation and advice, and each is followed by a plain infinitive. Poder is can, tener que is the everyday have to, and deber is the softer should or ought to. Only the modal changes for the person - the second verb never does.
Examples
- ¿Puedes llamarme más tarde?
- Can you call me later?
- Tengo que enviar un mensaje ahora.
- I have to send a message now.
- Deberías contestar a su correo.
- You should reply to their email.
The mistake to avoid
Tener on its own means to have; the obligation only appears with que - "tengo llamar" is wrong, it must be "tengo que llamar".
Where this fits in
Teléfono, mensajes y pequeños problemas, Spanish A2: Llamar y escribir, disculparse, aclarar un malentendido y pedir ayuda: la vida diaria cuando algo no sale como estaba previsto.