误There are six lesson period in a normal school day.
正There are six lesson periods in a normal school day.
析period作“学时、课时”解时有复数形式。
在那些动乱年代里,我们在农场呆了很久。误During those years of turmoil, we stayed on a farm for a long period of time.
正During those years of turmoil, we stayed on a farm for a long time.
析误句中的period of是多余的,应删去。