If a verb is followed by the aspect particle "了" and a simple object, a numeral + measure word compound or some other attributive or a more complicated adverbial is used before the object to make the sentence grammatical complete.

e.g. (1)妈妈买了一双皮鞋。(2)他穿了很漂亮的衣服。(3)他在会议上作了报告。