This is the 1275th most frequent Turkish word.
birden
"Birden" means "suddenly" or "all of a sudden" in English.
Here, 'birden' is used as an adverb meaning 'suddenly'.
Birden gökyüzü karardı.
Suddenly, the sky darkened.
In this sentence, 'birden' indicates an action performed in an abrupt manner.
Kapıyı birden açtı.
He/She opened the door abruptly.
In this context, 'birden' conveys the sense of 'more than one' or 'several'.