it was there that great Chairman Mao established the first rural base for the revolution which was led by the Communist Party of China.

【答】这是一个非常明显的强调句式。that great Chairman Mao established the first rural base for the revolution which was led by the Communist Party of China不是主语从句。

【翻译】就是在那里，伟大领袖毛主席为中国共产党领导的革命建立了第一个农村根据地。

如果把that great Chairman Mao established the first rural base for the revolution which was led by the Communist Party of China理解为主语从句，it是形式主语，那意思就讲不通了。

1. there是地点“那儿”（place），

2. “伟大领袖毛主席为中国共产党领导的革命建立了第一个农村根据地”是一个“事件”（event）

“事件”是“地点”？逻辑错误。这两者不可同日而语！