(I'm sorry if this is too technical.) "Tell" is transitive, doing an action /to/ someone or something. I tell you something. I told him something. She told me something. "Say" is intransitive, meaning it can stand by itself. He said. She said. They said. I said. "Tell" must have an object. Usually, these are indirect objects without a preposition. WRONG: "Don't do that!" I told. RIGHT: "Don't do that!" I told [to] him. Direct objects are usually the "what" is being told. "I don't trust politicians. They only tell lies." "Be wary of friends who tell only half-truths." "Lies" and "half-truths" are the direct objects. "They only tell lies to us." They (subject) only (adverb) tell (verb) [to] us (indirect object) lies (direct object) "They only tell us lies." "Say" does not need an object. "Of course not," they said. EXCEPTION: "Say" does not have an inherent object, but may take an arbitrary indirect object. Therefore: "Don't do that!" I told him. and "Don't do that!" I said to him. are both correct and mean the same thing. HOWEVER, if you want to add an indirect quote, you must use "tell." WRONG: She is saying to him not to play with his food. RIGHT: She is telling him not to play with his food. RELATIVE PRONOUNS may use either "tell" or "say." I said that I would fix it, and I will. I told him that I was going to do it, and I did. MORE EXAMPLES: I told him to go to the store an hour ago! I said [that] I wasn't going. All I'm saying is that we should consider this carefully. She told him that Cindy told her that Shawn told her what Kevin said to him last week. The book says [that] part A connects to part B.