如题所述
回答：一个句子必须有主语、谓语。从句也是句子，它跟主句一样也有主语、谓语。从句在句中可以作主语（主语从句）、状语（状语从句）、宾语（宾语从句）、表语（表语从句)、同位语(同位语从句)、定语(定语从句)。而非谓语动词(不定式、动名词、现在分词)则不能在句中作谓语。不定式在句中可以作主语、表语、宾语、定语、状语、宾补; 动名词在句中可以作主语、定语、宾语、表语、同位语; 分词(包括现在分词和过去分词)在句中可以作表语、定语、状语、宾补。故从句中必须有谓语，但并非必须有非谓语动词(有否非谓语动词跟谓语动词的用法有关，跟非谓语动词的用法有关。)。例如：
He is the man who I saw yesterday.
I know the man who is sitting in the chair watching TV.
英语从句中谓语动词的形式跟主语有关， 跟时态、语态有关。例如：
He says that Mary enjoys jogging in the morning.
The bridge which was built five years ago is 80 metres long.
He is the man who I saw yesterday.
I know the man who is sitting in the chair watching TV.
英语从句中谓语动词的形式跟主语有关， 跟时态、语态有关。例如：
He says that Mary enjoys jogging in the morning.
The bridge which was built five years ago is 80 metres long.
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