一、But用于句首吗？
①
是的，but用于句首
but属于连词。连词的作用就是连接两个完整的句子，所以一般来说and和but前面都是逗号，在严谨的书面语中情况是不用在句首的（口语例外）
不过有时候介词for可以和but搭配放在句首，意为：要不是，如果没有。
例如：But for your help, I couldn't have finished the work in time.
②
but是完全可以放在句首的。
Invented Rules很多时候只是一些人的偏好，一种风格而已。
作者说“For 250 years, grammarians have accused the best writers ofviolating rules like these, and for 250 years the best writers haveignored them.” 250年以来，优秀的作者们是不把这类规则当回事的。
这里就提到了but是否可以用在句首的问题：
牛津出版的Oxford Guide to Plain English中把“but不能放在句首使用”列为了6大写作误区（writing myths)之一，作者说：This is neither a rule of grammar nor even a widely observed convention.我们在阅读和写作的时候要注意观察，然后知道什么样的规则是必须遵守的，什么样的规则是没那么重要的。
在一个教雅思的网站上有人也讨论了but是否可以放在句首的问题：
Can we begin a sentence with and or but?
This is a frequently asked question. Although some grammarians might argue that it is improper to begin a sentence with and or but, this practice has been in use since Anglo-Saxon times. In fact, starting a sentence with these conjunctions might create a powerful or dramatic effect.
However, you should consider avoiding an initial and or but in cases where the sentence will do just fine without the conjunction at the beginning. In some cases, the sentence beginning with and or but and the one that comes before it can be connected into a single compound sentence.
He soon got addicted to gambling. And that didn’t surprise anyone.
They abused him. They threw stones at him. But he didn’t protest.
Beginning a sentence with a conjunction is essentially a matter of convenience. It doesn’t make your sentences ungrammatical, but avoid this style if you are writing for an audience who might object to it.
这里说but放在句首是可以的，但是要注意是为了强调的目的才这么用的，不要滥用。
Fowler's Modern English Usage中说“...unless contextual dislocation isbeing deliberately sought as a rhetorical device, it is not desirable to litter the pages with constuctions like He is tired. But he is happy.”
二、如何安全使用网络的英语作文？
To earnestly strengthen the education of network security knowledge for students, all teaching departments are required to actively open computer network courses according to the requirements of the national curriculum plan, so that students can master the necessary computer network knowledge.
The school network classroom should extend the opening hours as far as possible to meet the students' Internet requirements.
At the same time, it is necessary to teach students the necessary network security knowledge and enhance their awareness and ability of security prevention by adopting flexible and diversified forms according to the characteristics of students' ages.
We will further carry out the study and publicity activities of the National Convention on Internet Civilization for Youth.
Education students to be good at online learning, not to browse bad information;
Be honest and friendly, don't insult or cheat others;
To enhance the sense of self-protection, not casual dating net friends;
We should maintain cyber security and not disrupt cyber order.
Do not indulge in virtual space and time.
Strengthen the management of electronic reading room, multimedia classroom, computer room and other places where students surf the Internet, prevent reactionary, pornographic, violent and other unhealthy contents from harming students, guide students to treat the Internet correctly and surf the Internet in a civilized way.
We will strengthen the development and management of cybercivilization and cybersecurity.
Various effective measures should be taken to improve the ability of young students to distinguish between right and wrong, network moral level and self-discipline consciousness, as well as the ability to prevent injury and protect themselves in the network environment.
Pay attention to in the campus network construction process, establish network security measures, provide multi-level security control means, establish security management system.
Strengthen campus network civilized publicity and education.
All departments and schools are required to pay attention to the study of preventing the adverse effects of the Internet on teenagers, and actively guide teenagers to get on the Internet healthily.
We should strengthen the construction of teachers, so that every educator can understand network knowledge, abide by network morality, learn network laws and regulations, and conduct targeted education on network morality and network security to students through classroom teaching and extracurricular activities.
Increase exchanges and cooperation with relevant departments of national network security, strengthen the governance of the campus surrounding the Internet service business.
Actively cooperate with municipal departments of public security, fire protection, culture, industry and commerce, urban management and other departments to carry out supervision and management of Internet service business places inside and around the school, so as to prevent students from indulging in Internet bars.