He shouted "Watch out!" but it was too late; she had already knocked over the lamp.

watch 在这里是动词，意思是"注意、观察"。 out 是副词，表示"向外、离开"的意思。 watch out 是一个固定短语，意思是"当心、小心"，用于提醒别人注意危险或潜在的问题。