There are only a few words which you should pluralize by changing the **f** or **fe** to **v** and adding **es**. They are as follows:
 

 Singular ending in **f**

 - **calf** to calves
 - **corf** to corves  
 - **elf** to elves
 - **half** to halves
 - **hoof** to hooves
 - **leaf** to leaves
 - **loaf** to loaves
 - **scarf** to scarves
 - **self** to selves
 - **sheaf** to sheaves
 - **shelf** to shelves
 - **thief** to thieves
 - **wharf** to wharves
 - **wolf** to wolves
 - **yourself** to yourselves

Singular ending in **fe**

 - **knife** to knives
 - **life** to lives
 - **wife** to wives

[SpanishDict.com][1], from which I obtained most of this list, also lists *ourself* to *ourselves* and *themself* to *themselves*, but since *them* and *our* are plural, I don't believe *ourself* and *themself* are legitimate words.

So, keep this list. You could probably even memorize them. By my count, that's only 18 words. 

You must also remember that if a compound word ends with one of the words that uses the **ves** ending to pluralize, the compound word will do the same. For example, *knife* changes to *knives*, so *penknife* will change to *penknives*, and *wolf* changes to *wolves*, so *werewolf* will change to *werewolves*. There is a list at [en.wictionary.com][2] which includes quite a few of these kinds of compounds.

To pluralize the other words ending in **f** or **fe,** simply add **s**.


  [1]: http://www.spanishdict.com/answers/237034/forming-plural-nouns-in-english-part-2a-words-ending-in-f-or-fe
  [2]: http://en.wiktionary.org/wiki/Category:English_irregular_plurals_ending_in_%22-ves%22