Как правильно написать She plays the piano.(sometimes) She is very busy.(

She plays the piano.(sometimes) - She sometimes plays the piano.

She is very busy. (usually) - She is usually very busy.

В Present Simple наречия never, hardly ever, sometimes, usually, often, always в предложениях используются перед  глаголами (напр. play), но после модалных глаголов (напр. can) и форм глагола to be (am, is, are).

Наречие sometimes может употребляться в начале и в конце предложения. Напр.: Sometimes we go to the cinema.