Question

There is no doubt but he won.

but he won 是定语还是同位语？

2，It's not so high but we can jump it.
它的同义句是什么？

Answer 1

1. There is no doubt but he won.
此句中的but为介词，引导一个省略了that引导的宾语从句。
即 = There is no doubt but that he won.
毫无疑问他赢了。
当然，也有的语法书上直接把but等同于that，这样它就引导doubt的同位语从句了。

2. It's not so high but we can jump it.
= It's not so high that we can't jump it.
but = that...not...
它并没有高到我们爬不上去。

发现楼主有死钻语法的倾向了，呵呵。
这些用法都太难了，平常考试几乎不可能会考得这么偏门的。

Answer 2

第一个句子是同位语
it's short enough to jump it