句1:We feel natural to communicate with our friends.
句2:The water is safe to drink.
第一句漏掉形式宾语 it,不定式为真正宾语,natural 为宾补。应为:We feel it natural to communicate with our friends. 是不是这样,请查原句!
第二句, to drink 为反射不定式,其逻辑宾语为句子主语。这个句子可以有两种划分:
1. The water(主) is (系)safe to drink.(表)
2. The water(主) is(系) safe(表) to drink.(形容词补足语或状语)