關於英語為母語的英語教師（簡稱外師）的教學對台灣學生有何影響是值得探討的議題。本研究採用半結構訪談研究方法，收集深入的質性資料，並以紮根理論分析訪談資料。研究對象為在台灣南部的四所科技大學任教之八名外師，以及在本研究期間八名全職非外籍英語教師、12名英語系學生，以獲得多面向的看法。研究結果顯示外師的教學對學生的EFL學習有其好與壞的影響。正面影響包括（1）外師的語言優勢有利於學生的英語學習，尤其是在會話技巧方面；（2）外師教學之特色在於採用活潑的教學方法，例如：有趣的活動，豐富的肢體語言和互動，鼓勵學生以目標語言進行口說及聽力活動；（3）在外師教室中學生發展出文化的意識和/或啟發。負面影響：學生的語言障礙、對外師的誤解、在外師的EFL課堂中無法流暢地回應，最嚴重的是，學生產生學習焦慮及壓力。外師與學生在文化上與期待上具有差異，導致了課堂中進行真正的溝通時遇到困難，有時還會產生緊張關係。學生的雙語和跨文化需求與大多數外師所採取的單一語言或「僅英語」教室政策相左。

The effects of NESTs' instruction on Taiwanese students are worth investigation. A semi-structured interview study was adopted to collate in-depth qualitative data, along with a grounded theory to analyze the data. The participants included eight NESTs, currently teaching at the time of the survey, from four technological universities in Southern Taiwan, coupled with eight full-time non-NESTs and 12 English major students to gain multiple perspectives. The results demonstrated that NESTs' instruction had both positive and negative effects on students' EFL learning. The positive effects included: (1) NESTs' language edge benefited students' English learning, especially in conversational skills; (2) active teaching methods were characterized by the NESTs' delivery, such as fun activities, rich body language, and interaction, which encouraged the students to undertake speaking and listening activities in the target language; (3) the students in NEST classrooms developed cultural awareness. The negative effects were as follows: the students' language barrier, misunderstanding of the NESTs, inability to respond fluently in the NESTs' EFL classes, and, most seriously, learning anxiety and stress. Gaps between the NESTs and the students in relation to culture and expectations also resulted in difficulties in real communication in class and sometimes tensions. Students' bilingual and cross-cultural needs were at odds with the monolingualism or “English only” classroom policy of most of the NESTs.