2) Task cycle (while-task, during-task): (1) Task: After understanding the text, students work in pairs to act out the dialogue with the monitor of the teacher. (2) Planning: Different groups of students are carefully prepared to perform their tasks to the whole class in the form of oral or written report. It can make students compare their own performance with classmates’ and broaden their horizons; (3) Report: Students report their tasks.

3) Language focus (post-task): (1) Analysis: Students analyze the presentations of other groups and give some advice; (2) Practice: Teacher gives some tests to make sure whether students have grasped the new knowledge. Practical activities include the matching games, right and wrong judgments, the team competitions, filling in the blanks, dialogues, role-play, retelling the texts and so on. At this stage, the students evaluate their work and analyze the language features, which are called the consciousness-raising activities, aiming for the useful language acquisition. (Willis, 1996: 39-115) 

3.2 Task Design Principles 

Nunan (1999) proposes five teaching principles of TBI

1. The Authenticity Principle

In the task design, task input materials used should come from real life. Meanwhile, scenes as well as specific activities to fulfill the tasks should be close to the real life in which students have more access to real language information. As a result, the language skills they use in the classroom or real life can be effectively applied.

2. The Form-function Principle

The biggest shortcoming of traditional language practice is the decontextualized language. Students may know different language forms, but can’t appropriately express the meaning and function. The form-function principle makes clear of the relationship of language forms and function, as well as the relationship of language and context, and enhances learners’ understanding of language appropriateness.

3. The Task Dependency 

This principle involves the relationship between tasks, task steps and procedures in the classroom. It tells how to make the design achieve coherence and fluency in teaching and logic during the implementation process. In class, the teacher arranges a series of tasks and every task represents a ladder, making learners to achieve higher levels of language proficiency. 源.自|吹冰,-论`文'网www.chuibin.com

4. Learning by Doing Principle

Students stimulate their potentiality through learning and practice. Learners master language use through continuous communications in the classroom, even though they still need to strengthen their grammar learning and vocabulary memory.

5. Scaffolding Principle

Scaffolding is a process to set up a situation where children can entre the topic easily and successfully. Then teacher pulls back it gradually and offers children enough space to develop and grow. (Bruner, 1983:60)

4. Application of TBI to Grammar Teaching in Middle School 

4.1 Application to Grammar Teaching

A. Experimental Hypothesis

The experiment assumes that task-based Instruction is suitable for Middle School English Grammar teaching. TBI offers a great number of chances to students to communicate and practice. As a result, task-based instruction is more useful for improving the efficiency of English Teaching than the classic teaching approach and can attain better teaching achievements.

B. Participants and Basic Information

The participants in this experiment were students in Grade 9 from Yushan Middle School in Kunshan. There were 42 students in Class 1, which works as the experimental class and 40 in Class 2, working as the controlled class. All of them have received at least six years’ courses of English at school. After three months of study, the infinitive would be tested as the target grammar, because it is challenging for Chinese students to master its usage.