1. Internet 只有一個，永远要用the
Network N. countable
Life- lives plural
E.g. In this day and age, according to the development of the internet, social networks have become more and more vital in our lives.
2. It 只用于前面的某个主体，若前句主体不明或太长，用this
E.g. In this essay, I will discuss whether this is positive or negative.
3. Have been
e.g. Since I have been in Australia, I have made a lots of friends.