Their water isn't good to drink.这里的to drink是什么成份,

问题描述:

Their water isn't good to drink.这里的to drink是什么成份,
我认为是定语,是说明喝起来不好.

是后置定语~~~ 能做后置定语的成分有副词、介词短语、不定式、分词和attributive clause