There are several words which don't follow the usual rule of plural of S OR ES, which means we can often use the incorrectly, but these are pretty normal words used in our day to day life more than once a day. You must know them atleast as these are daily used words if you use them wrongly in ielts or in front of an English speaker you may sound dumb and get embarrassed by someone so listen this podcast atleast once and keep them in mind.