-ly / -ally

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!