Context would be very useful. 悲しい - い Adjective 悲しむ - [trans.] Verb 彼は悲しいです - Describing him, he is sad. 彼は悲しんでいます - A verb or action, He is being sad. When you separate nouns, adjectives, and verbs, you can see the subtle differences. Once you learn more conjugations, you'll be able to recognize what the base word is.