dormir

In a sentence

• Ayer dormí muy bien después de un largo día.

  Yesterday, I slept very well after a long day.

  Here, 'dormí' is used in the past tense (first person singular) to describe an event that took place.

• Siempre trato de dormir al menos ocho horas cada noche.

  I always try to sleep at least eight hours each night.

  'Dormir' is in the infinitive form here, used to describe the act or intention of sleeping as part of a regular routine.

• ¿Por qué no puedes dormir ahora?

  Why can't you sleep now?

  In this case, 'dormir' is in the present tense (infinitive in the question structure), discussing the ability/inability to sleep currently.