The object following bǎ把 is the recipient of the action and it must be a definite or specific thing or person or a known entity. It usually has been mentioned earlier or is known to those engaged in the discourse, otherwise the listener would not understand which thing or person the speaker is talking about.
1. Tā bǎ nèi liàng jiù chē mài le.
She sold that old car.
‘That old car’ is one that is known to both parties in the discourse.
2. Jīntiān wǒ bù xiǎoxīn bǎ yí ge nánde
Today I knocked down a man by accident.
‘A man’ here refers to a specific one, the one that ‘I’ knocked down.
3. Qǐng bǎ táng dìgěi wǒ.
Please pass me the sugar.
‘Sugar’ here refers to something in the presence of both parties. It is something that they both can see.