Some nouns ending in -f or -fe have plural forms ending in -ves. And some don't! As ever, there are always exceptions in English to keep you on your toes.
What happens to words like "knife", "wife", "half", "elf" and "life" when they become plural? If you say the words aloud, you notice that the f begins to soften and becomes a v sound instead. In spelling, these plural words look like this: "knives", "wives", "halves", "elves" and "lives". In every case, the v is followed by an -es, rather than only an -s ("knivs" just wouldn't look right somehow, at least in English).
Knowing how to make words ending in -f or -fe plural is one of those useful spelling rules to learn. If in doubt, try saying the plural form aloud, listening for a v sound. If you can hear the v, the word is not likely to be an exception to the rule. These exceptions include "chief" (chiefs), "roof" (roofs) and words ending in a double f, such as "cliff" (cliffs, not clives....).
Spelling is such an important skill, so why not play all of our spelling quizzes to really sharpen your mind!