"I heard him drop his keys"
I = subject
heard = verb
"him drop his keys." = direct object.
But how can I understand 'drop his keys'?
"I heard him singing in the shower."
Here, "singing in the shower" is an adjectival participle phrase.
"I wanted her TO sing in the shower."
"to sing in the shower" is an infinitive phrase.
But in 'him drop his keys", drop is not a participle, and it's not quite an infinitive with an understood 'to' either (who would say, "I heard him to drop his keys"?) and it's not a truncated phrase like "as he dropped his keys."
"drop his keys' answers the question 'what?" which is a noun sort of question, as opposed to the adverbial "when?", "how?, "where?"" questions, or the how many?", "what kind of?" adjectival questions.