I do agree with Halliday that the second and first language learning is similar as far as the goal of the learning is concerned. As for the means to the goal, even though there are some similarities in the process, learning the first and second language is far more different rather than similar. The main reasons for this are that the second language learner has already had language background-knowledge of his mother tongue, and that we cannot create a second language learning situation to be any sense like learning the first language in the real-life situations.
The knowledge of the child’s first language development has and obvious influence on second language teaching and learning. One thing that is directly influenced by the knowledge is the contents of the second language courses. If we believe that the child develops in him, in order to acquire his first language, language rules, then, the contents of the courses will surely be about linguistic rules or grammar. The traditional approach of language teaching-the grammar-translation method, for example, assumed that the child learns his first language and uses it fluently and correctly because he has learnt the rules and applied them easily and effectively. So, in learning the second language, it is necessary for the learner to know the rules of grammar of the target language. The contents of the courses based on the approach, thus, are grammar or linguistic contents and nothing else but grammar.
Another thing that is influenced by the knowledge of a first language development is the method of teaching. This influence can be seen clearly through the behaviorist’s Audio-lingual approach basic ideas one of which is that we learn our first language by listening and speaking first, then we learn to read and write. The teaching of language, thus, must begin with listening and speaking, then, reading and writing respectively. Another basic idea is that language is learnt by a process of habit-formation in which the learner imitates the sounds and patterns repeatedly until these become their habits-that is to say, until the learner can imitate and is able enough to produce what he has learnt correctly and automatically. It is because of this basic idea that drills are extensively and intensively used in the teaching of second language through this approach.
So, we can see that the knowledge of first language development has shaded a lot of influences upon second language course contents and method of teaching. In other word, the knowledge of first language development is one of the important factors that determine “what” and “how” to teach in the second language courses.