Not all English nouns can be made plural only by adding an "s" to the end. These are called irregular plurals. Many words that end in "f", like "leaf", "loaf", and "calf", change their sound when they become plural: "leaves", "loaves", and "calves".

"The following list shows the ten nouns with irregular plural forms that cause the most difficulty for advanced learners of English. The words on the left show the singular form of the noun and the words on the right show the correct plural form: 1. life - lives 2. child - children