Hymyillessään
Hymyillessään is a Finnish adverbial participle form of the verb hymyillä, meaning to smile. It expresses that the action of smiling occurs at the same time as the action of the main clause, roughly translating to “while smiling” or “smiling, …” The form is used to provide background or tone to a sentence.
The participle is formed from the verb hymyillä with the adverbial -essaan/-essään ending, indicating third-person singular
Hän katsoi ulos ikkunasta, hymyillessään. (He looked out the window, smiling.)
Hymyillessään hän kertoi tarinan. (While smiling, he told a story.)
Hymyillessään is more formal and stylistic than simply using hymyillen, which describes the manner of smiling.
While the form is standard, its use is more prevalent in written than in casual speech. Correct