Why is production so much harder than comprehension?

Clark explains a study that showed the gradual process in which children learn to create new words that are in fact "accurate" words in the English language. This specific study was with the suffix -er and how children acquire the ability to use this device correctly. Clark mentions that kids are not the only people to create new words; adults do so as well, but have a larger and more sophisticated repertoire of words and word-formation devices. The three factors that affect word-formation device acquisition are semantic transparency, productivity, and conventionality. Transparency means that we use known elements, particularly ones with one-to-one meanings. The youngest children in the study exhibited behavior in alignment with this principle by relying on simple compound structures to combine two words that they already knew well. Productivity means that kids try to learn devices that are frequently used and specialized. This reflects the next stage of -er acquisition because children began to have productive control over the device for at least one of its meanings (the agentive meaning). Conventionality means that we prioritize a word or device over others if it is conventionally used. Older children learned to give up their own creation of words and replace them with the conventional word for that meaning. This stage was seen in the study when the oldest children had gained control over the second (instrumental) meaning of the suffix -er.

Clark also made an interesting distinction between comprehension and productive control. Despite the fact that three year olds can understand -er and distinguish the correct verb form given a question such as, "What does a stopper do?" they still cannot reverse-engineer to answer the question, "What could you call someone that stops?" This sparked my curiosity of the difference between language comprehension and production. I feel that I have experienced this distinction in my own life in regards to learning German. I have always been better at translating German to English, and not vice-versa; I could easily understand Germans when they spoke to me because my brain seemed to recognize the words and know their English counterparts. However when asked to produce a sentence in German on my own, I always struggled to think of the correct German word; but, as soon as somebody told me it, I would immediately say that I knew that! This separation between comprehension or recognition and actual production also makes me think back to the Lupyan article from a few weeks ago. Maybe kids are better at understanding -er before effectively applying the device because of different mental representations of the abstract concept of -er. Perhaps we store the device in two separate forms in our minds so that we can access its basic definition when presented with the first set of questions in the study, yet have a different abstraction of -er in our repertoire to be used when we need to construct a word that actually implements it.