Answer: The milk turned sour after sitting out on the counter over night.
Linking verb is a kind of verb that connects or links the subject to the predicate of the sentence without showing action. Basically, linking verbs are used in a sentence to describe or recognize again the subject. Some of the examples of linking verbs are is, was, are, and were.
In the sentence, the word "turned" is used as a linking verb because it does not show action. It only connects the subject which is the "milk" to the predicate which is the "sour." It is used to describe the milk.