In the sentence having main verb ‘know’, infinitive is not used just after it but is used after using appropriate question words such as how / when / why / where.
Example: He informed me / that he knew to / play on the harmonium. / No error
Explanation: As per the rule given above there is an error in the second part of the sentence, question word ‘how’ should be used after main verb ‘knew’. So, insert ‘how’ in between knew and to.
Therefore the correct sentence should be:
He informed me that he knew how to play on the harmonium.
Example: He informed me / that he knew to / play on the harmonium. / No error
Explanation: As per the rule given above there is an error in the second part of the sentence, question word ‘how’ should be used after main verb ‘knew’. So, insert ‘how’ in between knew and to.
Therefore the correct sentence should be:
He informed me that he knew how to play on the harmonium.