Correct spelling


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


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.

Incorrect spelling


Incorrect spelling