Correct spelling, explanation: this form is correct because the adjective sudden and adverb suffix -ly gives the word suddenly. There is no need to add any extra letters, so
suddendly is a misspelling and the only correct form is suddenly.
Definition of suddenly:
adverb, quickly, usually unexpectedly
Suddenly, the loud sound woke everybody up.
I was walking down the street and suddenly I realised I had forgotten my phone.
Incorrect spelling, explanation:
suddendly is incorrect because there is no reason to add d between root adjective sudden and adverbial suffix -ly. Therefore the correct form is suddenly. Sudden + -ly is suddenly.
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