I knew adverbs could not be as easy as I thought!
Well... congratulations, you were right. Not all adverbs are formed like the adverbs with -ment! But don't panic, some grammar rules are even less complicated!
1. When an adjective already ends with a vowel, you can simply add the ending -ment to it!
2. When an adjective ends in -ent or -ant, the ending of the adverb will become -emment and -amment.
That's still pretty easy... I may be getting the hang of this!
The adjective lent is an exception to the last rule. It's adverb is: lentement.