one L or two L's

[valtango]

Good morning oh wise ones. Can you give me a rule for the use of L or double L at the end of a word. If I am right, if one word is made from two words, you then use a single L. For instance Care and full when put together make careful, but of course it isn't that simple, because when you write carefully, the L is doubled again, is there a general rule about this? Hope you can help. Thankyou.

[rogusx]

I would have to say that the rule is that the suffix is always "-ful" with a single "l". When you're making an adverb from the adjective, you add yet another suffix, "-ly", so you end up with the double L.