。
She sat down on a hard chair.
Literal
She [topic-は] hard chair [in-に] sat.
Two small details. First, 座る ('to sit') takes the resulting location with に (the goal/landing-place particle), not で — even though sitting feels like an action, Japanese encodes it as 'come to rest at.' Compare ベンチに座る ('sit on a bench') with ベンチで休む ('rest on a bench'), where the static action takes で. Second, 堅い is the 'rigid / hard / firm' kanji from a small family — 硬い (also 'hard, stiff,' typically for materials), 固い ('hard, solid; firm in resolve'), and 堅い ('hard, sturdy; reliable'). For furniture they overlap; the JIS-recommended modern spelling for a hard chair is 硬い, but 堅い is far from wrong.