He went to the market (to) buy some vegetables.

He went to the market (to) buy some vegetables.这个to是不定式作状语,表示目的,为了.
The government warned some factories not (to)pollute the water again.这个to是用在warn sb （to） do sth 做宾补 警告某人（不要）做某事 另外,pollution应该是pollute 动词