帮忙看一下这句话The Dragon Boat Festival, also called the Duanwu Festival

楼上说的不够详细

The Dragon Boat Festival, also called the Duanwu Festival．
这句话是对的，译文为：龙舟节，又称端午节，……
也就是说，上面这句话只是一个名词性短语做主语，后面还需要跟动词。

如果这句话加上is,即变为The Dragon Boat Festival, is also called the Duanwu Festival．
也是一个正确的句子
译文为：龙舟节，又被称作端午节。
这已是一句完整的句子，只是日常表达中，这句话中间不需要加逗号，即The Dragon Boat Festival is also called the Duanwu Festival．

错，没谓语的；
also前要加is的做谓语，要是also called the Duanwu Festival你做同位语的话，那么句子就出现逻辑错了，没谓语和其他部分。

这句话要有","就不能加is,它本身也不能称为一句话,而只是一个独立结构.要成为一个完整的句子,去掉","后再加上is 就对了.

楼主断章取义了，原文为The Dragon Boat Festival, also called the Duanwu Festival，is celebrated on the fifth day of the fifth month according to the Chinese calendar.其中also called the Duanwu Festival为同位语。