There is simply not enough stimuli to learn incorrect structures. Even so, the child still produces new correct structures which haven't been taught. Somehow, the kid knows how to string together correct sentences without learning them through some kind of manuel. Something is unarbitary when an object is used to represent itself. This object may have specific features that others wouldn't know, in this way it is unarbitary.
Language works with the issue of the arbitariness of the sign.
Words strung together are used to communicate a meaning. If we had no knowledge of what 'things'mean then language wouldn't be able to communicate them. The fact that everyone has a concept of what things mean allow language to strive by universal knowledge. Symbolism then leads to a functional advantage of acting out ,to saying words. Mimmicking can be passed on culturally rather than biologically.