这句英语,主语是topics吗?主谓宾或者主系表都是什么,从句?Topics such as death,for example,were once considered so upsetting and unpleasant that it was a taboo to even talk about them.

Topics(主语） such as death, for example,were（系） once considered （表）so upsetting and unpleasant that it was a taboo to even talk about them.
so...that...引导结果状语从句

严格的说，这个句子没有主语。主语只限定在单句中，而整体上看这个句子是so……that复合句。so前面的是说原因的Topics such as death, for example,were once considered so upsetting and unpleasant 这可以算作主语。而在这个句子中Topics such as death, for example是主语were once considered 是系动词，upsetting and unpleasant 是表语，that后面的 it was a taboo to even talk about them. 是一个动词不定式it+系动词+ sth+to do
整个句子的译文是：许多话题，例如关于死亡，都是如此令人伤感以致讨论他们成了一种禁忌

Topics such as death, for example,were once considered so upsetting and unpleasant that it was a taboo to even talk about them. 整个句子直译过来大意是：诸如像死亡这样的这样的话题曾经被认为是如此的令人...