Simple:
* actual - actually
* casual - casually
* formal - formally
* general - generally
* initial - initially
* personal - personally
* usual - usually
More Complex:
* annual - annually
* continental - continentally
* essential - essentially
* experimental - experimentally
* financial - financially
* individual - individually
* manual - manually
* mental - mentally
* natural - naturally
* original - originally
* potential - potentially
* professional - professionally
* social - socially
* spiritual - spiritually
* statistical - statistically
* substantial - substantially
Note: Not all words ending in "l" can take the "-ly" suffix. For example, "cool" does not become "coolly."