However, the study of Schitt&Meara(1997) demonstrated that the L2 proficiency had no influence on the acquisition of suffix。 There was no significant correlation between the tested Japan students’ suffix knowledge and their L2 level (TOEFL score)。

From the above researches and studies, we can infer that there are some relations between WDK acquisition and English proficiency, but accuracy is still waiting to be verified。

2。3 WDK and its influence on Language proficiency

Language proficiency can be demonstrated in many aspects。 The Common European Framework of Reference for Languages (CEFR) distinguishes between four kinds of language activities: reception (listening and reading), production (spoken and written), interaction (spoken and written), and mediation (translating and interpreting)。 More generally, it can be reflected in four aspects: writing, speaking, reading, and listening。 

Following are some studies about the relation between WDK and the Language proficiency:

2。3。1 WDK and Writing proficiency

S。B Cardoso，D。D。S Leandro，F。V。D Paula (2008) suggested that children already had knowledge that the morphological and maintains positive relationship with the performance in the writing。 They investigated whether the children of the 1st and 3rd grade elementary school have knowledge morphological derivational, and what its relation to performance in written spelling of words alone。

Followed by Dmitri Leontjev, Ari Huhta and Katja Mantylab (2016), they also did a research about “the link between Word derivational knowledge and writing proficiency”。 In their study, 117 10th grade learners of English in Estonia and Finland were administered two writing tasks as well as nine measures which were hypothesized to tap learners' word derivational knowledge。 The findings indicated that the learners' performances on almost all WD measures were significantly and strongly correlated with their writing proficiency。