它在这个句子中是主语补足语。

It is the subjective complement in the sentence.

它在这个句子中是主语补足语。

There is a straight line under the sentence.

句尾表示结果的形容词或介词短语可作宾语补足语或主语补足语，或描述运动实体的终态，或描述运动的路径。

The resultative AP or PP in the end position, which functions as object-or subject-complement, depicts either the end state of the entity in motion or the path of the motion.

（ 主语补足语）人们发现电是有用的。

Electricity is found to be useful.