Lǎorén xiǎng zhàogù sūnzi, dàn niánjì dà le, yǒuxīnwúlì.
The old man wants to look after his grandson, but at his age he's willing yet unable.
Tips
history
From the Southern Dynasties text 《高僧传》 by Huijiao (Liang dynasty): "贫道必当尽诚上答，正恐有心无力耳" — a monk fears he cannot repay a kindness because, though willing, he lacks the strength.
usage
Often used to politely decline or apologize. Compare 力不从心 (near-synonym, slightly more about mental/physical incapacity) and 爱莫能助 (wanting to help but unable to).
