Sudden has been used as an adjective since the 15th century, and it was once a noun, too, meaning "that which is sudden." Today the noun form is obsolete, except in the phrase "all of a sudden."

Is sudden an adjective and adverb?

As detailed above, 'sudden' can be an adjective or an adverb. Adjective usage: The sudden drop in temperature left everyone cold and confused.

Is suddenly an adjective or verb?

Happening quickly and with little or no warning; in a sudden manner. "Suddenly, the heavens opened and we all got drenched."