Here are some examples of how "plaintive" can be used:
* The wind carried a plaintive cry across the empty field. (The cry was sad and mournful, but not loud or harsh)
* The musician's plaintive melody tugged at the heartstrings. (The music was filled with sadness and longing)
* She cast a plaintive look at her lost puppy. (Her expression was sad and wistful)
Think of it as a softer, more subdued version of "sad" or "mournful."