## 6 Conclusions  
This paper has adopted inductive grammar approach to investigate the syntactic, semantic parameters of _OTC_ in L1 student essay writing. With corpus-based techniques, the frequency of _OTC_ associated with negation in a preceding clause-complex was identified, which serves the basis of the distinction between the two functional types reported in this study. It has been shown that more than 50% of the instances of _OTC_ in the corpus are not associated with a preceding negation. This seems contradictory to the descriptions of the expression found in most dictionaries and studies on adversative discourse markers which either explicitly or implicitly indicate the principle of negation. However, instead of simply reporting these instances as errors, the analysis in this study argues for an adversative use of _OTC_ which represents the dominant usage of this expression in the corpus. This suggests that L1 corpora could serve as an alternative resource that provides L2 learners with better direct information than dictionaries which tend to provide strict rules and assumptions about the context of use of a specific word. Indeed, the technique of using L1 corpora in developing learning materials in L2 contexts has been advocated by many researchers [15, 16]. The attributes of topical themes and themes of clause-complexes connected in the conjunctive relation were analyzed in order to interpret the semantic features of adversative and replacive _OTC_. Equivalence of topical themes of clause-complexes connected, in conjunction with negation serves the distinctive feature of replacive _OTC_, while the adversative type is characterized by different topical themes as the first pair of contrast and different themes as the second pair of contrast. In addition, unlike the replacive _OTC_ with one single focus, the adversative _OTC_ is associated with two-fold contrast and hence a divided focus. It is important to note that although in previous studies the theme-whence structure of clause has been applied to investigate the information flow of text in the academic genre to provide guidance to L2 novice writers [17, 18, 19], it has rarely been used as a descriptive framework in analyzing linking adverbials. The thematic analysis conducted in this study provided not only a descriptive framework for characterizing the distinctive features of two types of _OTC_, but also a means of highlighting the textual function of the conjunct. The thematic analysis that characterizes different semantic restrictions of two types of _OTC_ is further supported by the syntactic analysis of the conjunct. While clause-initial _OTC_ can be used to encode either an adversative or a replacive relation, instances of post-subject non-clause-initial _OTC_ are deployed to encode an adversative relation, in which different topical themes are highlighted in initial positions of their respective clause. Pedagogically, the findings would suggest the adoption of an approach which highlights the syntactic and semantic features of conjunctive adjuncts such as _OTC_, rather than presenting them as defined by grammar books. The findings of the present study are especially relevant to EFL learners who wish to acquire native-style writing. Despite the in-depth analysis conducted in this study, some limitations of the study should also be mentioned. First, the number of instances of _OTC_ in the corpus examined is relatively small. The limited use is mainly due to writers' preference for other more general markers of contrast, such as, _but_, and _however_[10, 20]. In future studies, it would be more desirable to examine the association of _OTC_ and negation in different corpora to test whether the patterns identified in this study can be generalized to other groups of L1 writers. Another limitation has to do with the identification of negation which is mainly based on a query of 10 grammatical words, such as not. The relatively small number of instances of _OTC_ also allows manual identification of negation.