How can I help my child learn to convey important things like "help," "stop," and "it hurts"?

These are top-priority words to grow, directly tied to safety. If words are hard, signs, picture cards, photos, or devices are fine. First, in an actual moment of trouble, the adult adds the words—"that's 'help'"—and responds right away, building the experience "telling gets help." Keep the cards always within reach. Practice as play in calm situations, and recognize it greatly when used in daily life. Whatever the means, put "getting through" first.