предложение с модальным глаголом : His wife Anna Petrovna could not work at the plant or at an office because they had three children.
В форме Past оно уже стоит
Форма Present His wife Anna Petrovna can not work at the plant or at an office because they have three children. (had меняется на have потому,что Present не согласуется с Past)
Форма Future : His wife Anna Petrovna will not be able to work at the plant or at an office because they have three children. (tp be able to - заменитель can в будущем времени)