将主题句置于文章开头是最常见的做法，这样可以开篇点题，为全文奠定基调。例如，在一篇关于“如何提高英语学习效率”的文章中，可以这样写：

主题句：To begin with, mastering effective learning strategies is essential for improving English proficiency.
后续内容：Firstly, setting clear goals helps you stay focused and motivated throughout the learning process. For instance, aiming to expand your vocabulary by 50 words per week gives you a tangible target to work towards. Secondly, utilizing a variety of resources, such as textbooks, online courses, and language apps, exposes you to different learning materials and keeps you engaged. Moreover, practicing listening, speaking, reading, and writing regularly reinforces your language skills and ensures a well-rounded understanding of English.
这种结构让读者一目了然地知道文章将要讨论的主要内容，即提高英语学习效率的关键在于掌握有效的学习策略，后续内容围绕这一主题句展开，详细阐述了具体的策略和方法。

文章结尾
将主题句放在文章结尾也是一种可行的方式，这种方式通常用于总结全文，再次强调文章的核心观点。例如，在一篇论述“大学生应该积极参与社会实践”的文章中，可以这样写：

正文内容：Social practice offers numerous benefits for college students. Firstly, it provides real-world experience that complements classroom learning. By participating in internships or volunteering projects, students can apply their theoretical knowledge to practical situations, enhancing their understanding of various industries and professions. Secondly, social practice fosters essential skills such as communication, teamwork, and problem-solving. Working with diverse groups of people and facing real-life challenges helps students develop these skills, which are highly valued by employers. Additionally, social practice promotes personal growth by exposing students to different cultures, perspectives, and environments.
主题句：In conclusion, engaging in social practice is crucial for college students to gain valuable experience, develop essential skills, and achieve personal growth.
这种结构在文章结尾处通过主题句总结了大学生参与社会实践的重要性和多方面的益处，使读者对文章的核心观点有更深刻的认识。

主题句原则在四六级写作中的应用能够帮助考生清晰地表达文章的中心思想，使文章结构更加严谨，内容更加连贯。考生应根据文章的具体内容和写作目的，灵活选择将主题句放在文章的开头或结尾，并确保全文围绕主题句展开，以提高文章的逻辑性和可读性。在备考过程中，考生应多加练习主题句的撰写和运用，以提升写作水平和考试成绩。