Question

What's the drop or keep the 'e' rule?

Answer 1

We usually drop the 'e' at the end of words when adding an ending that begins with a vowel suffix (e.g. -ing, -ed, -er, -able, -ous, ).But with words ending in 'ge' keep the 'e' before 'able' to keep the soft 'ge' sound-
manageable, changeable, knowledgeable...
We also keep the 'e' in words ending in 'ce' before 'able' to keep the soft 'c' ( "s") sound-
noticeable, replaceable, serviceable...
Another exception to the rule is the final -e is not dropped from words ending in: -ee, -oe, -ye.
see - seeing, agree - agreeing, canoe - canoeist, dye - dyeing