Chinese grammar is discourse-oriented, while English grammar is sentence-oriented. In Chinese grammar, it pays more attention to the whole topic and it is topic-prominent, the context and clauses are penetrated by the same topic. There is no subject-subordinate relationship between the main clause and the sub-clauses, but a chaining relation. Therefore, ‘因為’ and ‘所以’can appear in the sentence at the same time.
However, all sentence components should appear in the same clause in English, and one sentence describes one event. Therefore, there is a clear division between the main clause and sub-clause, and there can be only one main clause, which is subject-prominent. ‘Because’ is a subordinating conjunction that leads to a causative clause, while ‘so’ is a parallel conjunction that connects two simple sentences to make them parallel. If ‘because’ and ‘so’ appear at the same time, it is equivalent to having two conjunctions in two sentences, and the sentence will become a structure of ‘a subordinate clause introduced by ‘because’ + a parallel clause connected by ‘so’’, having a subordinate clause but no main clause, which causes a contradiction.
For example, we could not say ‘Because today is her birthday, so I bought her a gift.’. ‘Because’ and ‘so’ should be used separately - ‘Because today is her birthday, I bought her a gift.’ or ‘Today is her birthday, so I bought her a gift.’.
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劉美君. (2012). 英文文法有道理!: 重新認識英文文法觀念. 聯經出版事業公司.