「Adjectives and Adverbs」
「Adverbs Expressing Time」
Sudeni indicates an action or event that has been completed by the time some other action or event occurs.
Kūkō ni tsuita toki, hikōki wa sudeni dete ita.
When I arrived at the airport, the plane had already taken off．
Hoteru ni denwa shitara, kare wa sudeni chekkuauto shite ita.
When I called him at the hotel, he had already checked out.
Hope this helps!
°˖ ✧◝(○ ヮ ○)◜✧˖ °こゆうきあいはら°˖ ✧◝(○ ヮ ○)◜✧˖ °