The gymnasium was being built when an accident happened.这是并列句，when=and at that time. 如果非用状语从句，可译为An accident happened while the gymnasium was being built.
一般形式（两个句子）： The gym is building. An accident was happened. 复合句（时间状语从句）： The gym was building when an accident happened. 解析：因为“体育馆是正在建造”，也就是正在进行这个动作，所以用进行时；而“事故发生了”，也就是表示完成了，所以...
