Sentence: If I were the president, I would implement policies for climate change.
Explanation: The verb "were" is in the subjunctive mood, which is used to express hypothetical or contrary-to-fact situations. In this case, the sentence implies that the speaker is not the president, thus creating a condition that is not true. The subjunctive mood often uses the verb "were" instead of "was" in such hypothetical situations, which is a key indicator of its use.