you must keep the windows closed in winter 讲解谓语动词的特点 同类的动词还有

谓语动词一般在主语之后,由简单动词或助动词加上主要动词构成,如：
What happened?
I doubt it.
He worked hard all day.
The plane took off at ten o'clock.
这里的谓语动词,后面需接一个宾语和一个宾语补语,这个宾语补语是用过去分词充当的,又如：
I had my hair cut.
I heard the song sung in Spanish.
I heard English spoken.
希望我能帮助你解疑释惑.

谓语动词往往后面接名词或名词性的短语，双重宾语。
You must KEEP the windows closed in winter.中 keep 为 谓语动词， the windows closed 是一个双重宾语。