Some words in English do not follow the regular patterns of making nouns plural. Normal plural formation is done by attaching -s or -es to the noun, e.g. boys, houses, cars, books, ladies.
However, some nouns form their plurals by changing one of the letters in the word. Some also change the sound of one of the letters. These words are mainly of foreign origins.
focus. focuses or foci
appendix appendixes/ appendices
Rewrite the sentences below making the words in brackets plural.