A child is a competant user of language long before she has had the opportunity to hear enough of it to have learned it by mimicry or by explicit learning of language rules and words. When a child begins talking, she uses a language which is idiosyncratic and conspicuously different from her mother's, so there is no question of a straightforward playback. Shortly after birth babies are found to acvtively discriminate between sounds that will later make a difference to the meaning of words, but which at the time the baby knows nothing about. A baby will do this in whichever language she is born into, long before she has any reason to do so. A baby will, for example, tell the difference between 'sad' and 'sat' long before she knows what the words mean.
All natural languages are suspiciously alike at some deep level.
For instance, all are able to express in some way the concept of subject , verb and object . It is this deep-level identity that enables one language to be translated into another. All this has led linguists to believe that a newborn child has an innate capability for language. We might even say that a child is programmed to acquire any of the world's natural languages. Although study of how children acquire language may not fully explain second, as opposed to first, language learning, it has raised some important questions about the similarities and differences.