In the sentence "سمعت بكاء طفل," the word "بكاء" is a noun (اسم) that functions as the direct object (مفعول به) of the verb "سمعت" (I heard).
إعراب كلمة "بكاء":
اسم (Noun): منصوب (Manṣūb) وعلامة نصبه الكسرة الظاهرة على آخره لأنه اسم مجرور بحرف الجر "باء" (bi) الداخلة على الفعل "سمعت".
مفعول به (Direct Object): لفعل "سمعت" (I heard).
Breakdown:
اسم (Noun): "بكاء" (Crying) is a noun because it denotes a thing, which is the act of crying.
منصوب (Manṣūb): It is in the accusative case (Manṣūb) because it is the direct object of the verb "سمعت" (I heard).
علامة نصبه الكسرة الظاهرة على آخره (The sign of its accusative case is the visible dammah at its end): The dammah at the end of "بكاء" indicates that it is in the accusative case.
لأنه اسم مجرور بحرف الجر "باء" (bi) الداخلة على الفعل "سمعت" (Because it is a noun that is جرّ (majرūr) by the preposition "باء" (bi) that enters into the verb "سمعت"): The preposition "باء" (bi) before the verb "سمعت" requires the noun following it to be in the accusative case.
Therefore, the complete grammatical analysis of the word "بكاء" in the sentence is "اسم منصوب مفعول به" (Noun, accusative case, direct object).