The sooner you accept that English spelling doesn't make sense, the happier you'll be ;-)

The first person to answer was correct. Some words in English really make no sense and there's no reason for them to be spelled that way. Us native speakers don't get it sometimes either... haha.

Nouns on -o form the plural by adding -s or -es. add -s substitute with -ves singular plural singular plural a disco two discos a tomato two tomatoes a piano two pianos a potato two potatoes a photo two photos a hero two heroes Some nouns can have two plural forms: buffalo - buffalos/buffaloes mosquito - mosquitos/mosquitoes tornado - tornados/tornadoes There is no rule when to use -s or -es. We often add -s with technical words. .. Source: http://www.englisch-hilfen.de/en/grammar/plural.htm