In this level, we focus on two rules for making words plural.
First, if a word ends with a consonant and then the letter y, to make the word plural, the y typically changes to an i and then the letters e and s are added. So story becomes stories, but toy just becomes toys like normal since there isn't a consonant before the y.
Second, for words ending with f or fe, sometimes the f changes to a v and then an -es ending is added. So shelf becomes shelves and knife becomes knives. Rulebreakers exist, though: roof becomes roofs instead of rooves, for example.