Confused about how to properly spell an adverbial use of a word? Here's another simple trick -- so simple you won't care whether it's an adverb or an adjective!
If there is a form of the word ending in -al, then the proper use is -ally. If not, use -ly. That still sounds confusing, so let's look at some examples.
Incident -- incidental is a perfectly good word, so its correct adverb form is incidentally rather than incidently. Same for coincidentally, correct because coincidental is indeed a word.
Accidental makes accidentally your choice rather than accidently
A couple more examples I'm sure you already know:
Personally, not personly.
Independently, rather than independentally.
See? Using the correct terminology isn't hard at all!
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