While "thought" is the root word, there's no true "base word" in the traditional sense. "Thoughtful" is a derivative of "thought" formed by adding the suffix "-ful," which indicates "full of" or "characterized by."
While "thought" is the root word, there's no true "base word" in the traditional sense. "Thoughtful" is a derivative of "thought" formed by adding the suffix "-ful," which indicates "full of" or "characterized by."