The baby is sleeping是主谓结构。主语是：the baby.谓语是：is sleeping.英语中的实义动词有及物动词和不及物动词，及物动词后面必须要跟宾语，不及物动词后面可以不跟宾语，不及物动词后面如果要跟宾语，动词后面必须要加适当的介词。凡是“主语+不及物动词”这种结构的句子都是主谓结构的句子。例句：The bus has arrived.The train has left.It is raining.The wind is blowing.He is swimming.
