They+believe+that+you+can+do+it,哪个是主句哪个是从句
They believe ...是主句，you can do it是从句。
这是一个宾语从句，从句的引导词that之前的部分是主句，that不充当成分，也无实际意义，that之后的句子称为从句。
分类:宾语从句分为三类:(1)动词的宾语从句,介词的宾语从句和形容词的宾语从句.
时态: 1·主句用一般现在时，从句可用任意时。 2·主句用过去时，从句用过去某个时态。 3.当为客观事实和科学真理时，从句为一般现在时。Do you know who has won this game?你知道谁赢了这一局游戏吗？Could you please tell me how you read the new panel? 你能展示给我怎么用这个新的操作盘吗?
(2)宾语从句的从属连词主要有that,if,whether.that引导表示陈述句的宾语从句,而if和whether引导表示"是否"的宾语从句. He told me that he would go to the college the next year 他告诉我他将在下一年上大学. I don't know if there will be a bus any more. 我不知道是否还会有公交车. Nobody knew whether he could pass the exam. 没人知道他是否会通过考试.
连接代词主要有who, whom ,whose ,what ,whoever ,whomever ,whosever, whatever, whichever等.连接代词一般指疑问,但what, whatever除了指疑问外,也可以指陈述.I have found out that all the tickets for the concert have been sold out. 我发现这场音乐会的所有票都卖光了. Can you work out how much we will spend during the trip? 你能计算出这次旅行我们将花费多少钱吗?
主句是：They believe +宾语（that从句）.
从句是：you can do it.
that是引导词，在这里可以省略。
that 引导词
you can do it. 从句
意思是 他们相信你能做这件事。
