Aquellos montes están eternamente cubiertos de nieve: Those mountains are perpetually covered with snow.
The following simple and true rules will enable the student to understand the difference between ser and estar and to apply them correctly.
Ser is used—
1. To form the Passive Voice, as—
El cartero trajo las cartas—Active: The postman brought the letters.
La carta fué traída[114] por el cartero—Passive[115]: The letter was brought by the postman.
[Footnote 114: The Past Part. following "Ser" and "Estar" agrees in gender and number with the subject of these verbs.]
[Footnote 115: Este cuarto es barrido todos los días (this room is swept every day) is passive voice, because we speak of the action of sweeping, viz., somebody sweeps the room every day. Este cuarto está barrido—no voice; "barrido" is used as an adjective to denote state or condition.]
2. To denote an inherent[116] quality, as—
La nieve es blanca: Snow is white.
El hombre es mortal: Man is mortal.