They got caught when they ran across the border.

参考译文：当他们跑过边境时，他们就被抓住了。

请问 caught 是什么词性，在句子中的成分；请问单独一个分词，能作状语，表语，定语，宾补吗？ 

解答如下：

(1)caught 这里是“动词原形catch的过去分词”

 He did not like to be caught out on details.  

他不喜欢在细节上被人抓住把柄。

If he is caught again he will be given a custodial sentence.  

如果他再次被捕，将会被处以监禁。

Any child who is caught will be kept in at playtime.  

凡是被抓住的孩子在课间游戏时要被留在屋里。

She looked as if the photographer had caught her by surprise.  

看上去，她好像被摄影师搞了个猝不及防。

(2)单独一个分词是能作状语，表语，定语，宾补的。

A: Tired, he wanted to have a rest.（过去分词作状语）

B: He was tired now.（过去分词作表语,属于分词形容词用法）

C:This is a tiring job.（现在分词作定语，属于分词形容词用法）

D:I saw a pig killed.(过去分词作宾补）