Sentence: If I won the lottery, I would buy a house on the beach.
Explanation: This sentence uses the conditional mood because it expresses a hypothetical situation that is dependent on a condition being met (winning the lottery). The use of "would buy" indicates that the action of buying a house is contingent upon the fulfillment of the conditional clause "If I won the lottery." The conditional mood is often used to discuss possibilities or outcomes that are not guaranteed to happen.