because是连词，它后面只能接完整的句子？

The reason why people give up so quickly is because they look at how far they still have to go, instead of how far they have come.

老师，这句话是有语法错误吗？because应该改成that吧，instead of是复合介词，instead of how far they have come是介词短语作状语吧？

because只引导状语从句吧，它后面一定要加完整的句子，它还可以做并列连词吗？

【答】

第一问：语法书原先规定：主语是the reason，其表语从句只能用that 引导，不用because 引导（理由是：语法上不错，只是逻辑上重复）。但是，随着英语的发展，用because 引导也对，因为because 本身就可以引导表语从句。看来，这条规定过时了。

第二问：instead of how far they have come 是介词短语作状语吧？——介词之后接名词性从句，完全可以。是的，是作状语。

第三问： because 只引导状语从句吧—— 除此之外，because 还可以引导表语从句。

以下句子中的because，都是引导表语从句：

The reason our schools cannot afford better teachers is because people here are poor.

我们的学校请不起好老师的原因是因为这里的人穷。

来自柯林斯例句

The reason is because its economy is so weak. 

原因是因为该国的经济太差了。

来自BBC

The reason I can't go is because I'm busy. 

我不能去的理由是我忙。

【说明】1,3句中，reason 后定语从句的关系词why，还可以省略。

If there are things we do not know, it is because we haven't learnt them.

有所不知, 盖未学也。

来自《现代汉英综合大词典》

All this fuss is because Carol is having a bad hair day. 

就因为卡萝尔今天不太顺心，所以弄得大家鸡飞狗跳。

来自柯林斯例句

If he scolds you, it is because he cares for you.

如果他骂你，是因为他关心你。