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I am referring to the usage as defined on zdic.net
用在动词后表示动作完成：改～。戒～
Does 掉 often have the connotation of falling or missing？ What is the difference between (verb)掉 and （verb)了 and（verb）掉了？
I've heard this in the negative （擦不掉）。 How does that differ from（verb）不了？
When 掉 is used as a verb complement (not a verb), it indicates that something disappears, is removed, is disposed of, etc. as the result of an action. Not really "falling" or "missing".
Some examples:
擦不掉：Something can't be removed or got rid of by rubbing or wiping. Maybe it's a stain on your shoes, and you're trying to rub it off with a cloth, but it can't be wiped away.
吃掉：To "eat something up". All of it is gone.
卖掉房子：To sell your house. After you sell it, it's gone. It's off your hands now.
忘掉：To forget something. After you forget it, it's gone from your memory.
忘不掉：To not be able to forget something, to be unforgettable.
掉 and 了 can be used as verb or adverb.
掉 (verb) means drop or fall.
了 (verb) [pronounced as liao] means finish or understand, but it is rarely used in speaking.
掉 (adverb) is used to show the result of the verb, eg. 吃掉 (show result, ate it).
了 (adverb) [pronounced as le] means already. In grammar of English Language can be interpreted as perfect tense. Eg. 吃了 (has eaten).
Thus, 掉了 in Chinese can be interpreted as has fallen.
P/S: In fact, these explanations are not enough to explain all situations because these two words have a lot usage.
