According to Harris (1969), two important components commonly shared by
speaking, listening, reading and writing, are vocabulary and grammatical structure. The
third component is phonology (sound system). Furthermore, vocabulary has a unique
quality that other components (grammatical structure and phonology) do not have.
Whereas grammatical structure and phonology are mastered early in the language learning
experience, speakers continually add to their vocabulary store (Robinett, 1978). Robinett
stressed that vocabulary helps speakers to continually improve in speaking, listening,
reading and writing, irrespective of the levels of language learning.