* For plural nouns: Most words ending in "f" or "fe" change to "ves" in the plural form:
* knife -> knives
* wife -> wives
* wolf -> wolves
* life -> lives
* For verb tenses: Some words ending in "f" or "fe" change to "ves" in the past tense and past participle:
* leaf -> leaves
* sheaf -> sheaves
* thief -> thieves
* For other situations: There are some exceptions and special cases. Here are a few examples:
* chief -> chiefs
* roof -> roofs
* safe -> safes
* proof -> proofs
It's best to check a dictionary or online resource if you are unsure about the correct spelling for a specific word.