I'm glad if you correct my essay

I am going to describe the advantages and disadvantages of a smartphone.

These days, more and more people have a smartphone because of its good points. Firstly, it has many useful applications, so it can be a helpful machine if people go to unknown places or walk about. It guides us to our destination and counts how many times we go there. Secondly, a smartphone’s screen tends to be bigger than a traditional cellphone's, so not only young people but also elderly or far-sighted people can read it. If you want to see the big picture, you can reduce it. Alternatively, when you feel that the text or figure is too small, you can enlarge it. It means that the smartphone is freely adjustable according to use. Finally, people can choose a favorite cover from a number of options. There are cute, cool, low price, expensive and tough covers, so people can select the best cover.

However, you must not forget that smartphones have serious problems. The most serious problem is that they(smartphones) can cause light pollution. Their(smartphones) light is too bright especially at night, so we will be bothered from sleep and some will suffer from insomnia. Not only that, some people spend too much time on them(smartphones). This can lessen our communication with others. Moreover, now that they(smartphones) are prevalent amongst especially the young therefore some people are addicted to them(smartphones) and cannot live without them(smartphones).*

To sum up, it is true that a smartphone has good points, but we should not use it too often.

Very well written!

*notice that here we talk about smartphones all the time. So we write 'they', 'their', 'them' .... It is very important to decide what the noun is. Once the noun is there, it does not change. Here it is 'smartphones', so the pronouns are 'they, their, them ...'