这是什么从句?①His suggestion is that we should stay calm这句话是什么从句?②

①that引导的从句是系动词is所带的表语,叫表语从句,这是名词性从句的一种.
系动词后面都是表语,合称系表结构.
②不对.改为This museum is the one （that） you visited a few days ago.
（that） you visited a few days ago是定语从句,修饰先行词the one .关系代词that指代the one（this museum）,在从句中作宾语.that在定语从句中作宾语时可以省略.
缺少the one的话,定语从句就没有它所服务的对象——先行词.
这句话还可以将先行词与关系合为一体（即what),说成：
This museum is what you visited a few days ago.

1是表语从句
2应该是定语从句 所以啊 ②This museum is the one that you visited a few days ago.

①表语从句，that引导的从句在句中做表语
②不对，改为This museum is the one （that） you visited a few days ago.
缺少the one的话，就没有先行词，句子就乱了。that课省略，因为是作visited的宾语