In English, we can use a number of prefixes on adjectives to form their opposites. The following pairs come to mind as examples:
| typical | atypical |
| hydrous | anhydrous |
| social | antisocial |
| honest | dishonest |
| legal | illegal |
| possible | impossible |
| active | inactive |
| regular | irregular |
| existent | nonexistent |
| kind | unkind |
In most cases, these prefixes are not interchangeable, so we do not have *unlegal or *apossible, for instance. Of course, fluent speakers have no problem in using the right prefix for the right word, as more than often, the negative adjectives are learned word by word. But that should not stop us from pausing and asking, “Are these prefixes randomly matched with the words?” The lesson from Why is probible not possible and possable not probable? tells us that while not everything has a reason, most things indeed do not come from nothing. So the question that we should really ask is “What are the differences between these prefixes?”