dead
The word 'morto' is used in Italian to describe something or someone that is no longer alive or functioning. It can be used as an adjective or a noun, and it is often used to describe the state of being deceased. For example, 'Il cane è morto' means 'The dog is dead'.
The cat is dead
In this case, 'morto' is used as an adjective to describe the state of the cat. The verb 'è' (is) precedes it to form the present perfect tense.
I'm sorry, my grandfather died last week
Here, 'morto' refers to the past event of the grandfather's demise. It's explaining a past event using the present perfect tense in combination with a time expression 'la settimana scorsa' (last week).
After the accident, he remained dead on the field
In this phrase, 'morto' is used as an adjective, to describe the state of a person (lui) after certain circumstances ('l'accidente'). 'Rimasto' is a past participle of the verb 'rimanere' (to remain), and it's used in combination with 'è' (is) to form the present perfect tense.