关于“它们”用英语怎么说的问题，根据上下文和语法结构，可以分以下两种情况：
作为主语
当“它们”指代复数名词时，可以用以下结构：
- What are they？
- They are...
例如：
- What are they？
- They are nerve nodes.
- They are all major theatrical productions.
作为宾语
当“它们”指代名词或名词短语时，可以用以下结构：
- What do they do？
- What is their function？
例如：
- What do they do in the ecosystem？
- What is their function in the body？
引导从句
作为关系代词时，可以引导主语从句、宾语从句等：
- What they said surprised everyone.
例如：
- What they said surprised everyone.
总结：
主语：What are they？
宾语：What do they do？
从句：What they said...
根据具体语境选择合适的结构即可。