What suffix causes a word to become plural noun?

The most common suffix that causes a word to become plural noun is -s.

Here are some examples:

* cat becomes cats

* dog becomes dogs

* book becomes books

However, there are other suffixes that can also pluralize nouns, and some nouns have irregular plural forms.

Here are some examples:

* -es: bus becomes buses, church becomes churches

* -ies: baby becomes babies, fly becomes flies

* -ves: wolf becomes wolves, knife becomes knives

* -ren: child becomes children

* -men: man becomes men, woman becomes women

Remember, the best way to know the plural form of a noun is to look it up in a dictionary or use a grammar checker.