Tā yǐjīng yǒuyuē zài xiān, bùnéng dāyìng nǐ de qiúhūn.
She already has a prior commitment and cannot accept your marriage proposal.
Tips
usage
A polite way to decline an invitation or proposal — literally 'have an appointment beforehand'. Particularly common in (1) dropping a social invitation politely and (2) the romantic / fiction context where someone is already engaged or promised to another. Less stiff than 已有婚约 'already engaged' but heavier than the casual 有事 'I have something on'.
register
Slightly formal — appropriate in business, hosting, or polite social refusals. For everyday casual decline, 我有事 (wǒ yǒushì) 'I've got something on' is more natural.
