to be, estar when location or temporary state is expressed, otherwise ser.
to be able to, poder.
to be about to, estar para.
to be accustomed to, acostumbrarse a, soler (used only in the present and the imperfect, rd. ch.).
to be astonished, pasmarse.
to be at one's ease, estar a sus anchas.
to be back, estar de vuelta.
to be better to, valer más.
to be born, nacer.
to be composed of, constar de.