。
She used to play the piano often after school.
Literal
[topic-は] often after-school piano [object-を] played [used-to-ものだ].
~たものだ is the nostalgic-past construction — recalling a habitual past action with a wistful tone, often translatable as 'used to.' 放課後 ('after school') is a fixed compound noun, the standard term for the time after the school day ends — a culturally important window in Japan, traditionally devoted to club activities (部活), study, or in this case piano lessons. ピアノを弾く is the standard collocation: 弾く is the verb specifically for playing string-and-hammered instruments (piano, guitar, violin), distinct from 吹く for wind instruments and 叩く for percussion.