Regular plurals (II) more rules
The plural of nouns ending with f or fe is sometimes formed by adding s: sometimes it is formed by changing the f or fe to v and adding es: roof,roofs; thief, thieves.
The plural of most nouns ending in o is formed by adding s: except following words always end in es: tomato, tomatoes; potato, potatoes; echo, echoes; hero, heroes; torpedo, torpedoes. ( check your dictionary)