Here are some examples:
* toy -> toys
* key -> keys
* valley -> valleys
* monkey -> monkeys
* baby -> babies
However, there are some exceptions:
* Some words ending in -y preceded by a consonant change the -y to -ies:
* baby -> babies
* fly -> flies
* story -> stories
* Words ending in -o often add -es:
* hero -> heroes
* potato -> potatoes
* tomato -> tomatoes
* Some words ending in -o, however, add -s:
* radio -> radios
* piano -> pianos
* photo -> photos
It's important to remember that these are general rules and there are always exceptions. If you're unsure about a specific word, consult a dictionary.