【题目】改错：He playing a toy train. __________________
【答案】He is playing with a toy train.
【解析】
句意：他在玩玩具火车。现在进行时的构成为：主语+be动词+现在分词，句中缺少be动词，因主语是he，所以be动词用is。play 是进行某项运动，或演奏某种乐器；表示和谁一起玩用play with，所以把playing改为playing with。故答案为He is playing with a toy train.