correct my essay please

The most important things is not to be punctual when you attend an important appointment. Being late is impolite to the parties whoever involved in the process. What can you do if you have to be late due to an unexpected accident? Firstly, you have to call the relevant party to explain the reason clearly. Then, you should apologize upon your arrival. Make sure you have to be careful in regards to the diction of the words.To avoid the trouble of being late, make a timetable to schedule all of your appointments and choose whichever see fit. Hence, you should attend the appointment on time. The possibility of being late for an appointment will definitely minimize if you apply all of the aforementioned methods.

When you are attending an important appointment, the most important thing[註most必然是唯一最的-所以必定是單數的名詞] is not to be late[已有建議用be punctual]. Being late is an impolite to others. It gives the impression you are fooling around.[fooling around兒戲]

But if you late because of an accident. What can you do? First, you have to call the person you are meeting to explain clearly. Then, when you arrive[], you must speak 'sorry' to him/her. Be careful of your choice of words.

So, being late is always be perceived as unfriendly[]. To avoid of this trouble, set a timetable for all your appointments in the first place. Next, you should attending to the appointment as early as [] possible. []Do these, the problem of being late can be reduced.

When you attend an important appointment, the most important thing is not to be late, since being late is impolite.

However, if you must be late due to an accident, what can you do? First, you must call the relevant person to explain clearly. Then, when you arrive, you must say sorry to him/her, and choose your words wisely.

Therefore, being late is unfriendly. To avoid this trouble, plan a timetable to ensure the time is suitable for you. Next, you should arrive at the appointment as early as possible. By following these steps, the occurrence of being late can be reduced.

我盡量只改一些錯字或文法上的錯，因為句字結構都不錯，亦保存些你自己的寫作風格。

1. (when you attending...) Verbs ending with -ing is "present participle" and must always follow "is/am/are". So correct verb should be "are attending". In your essay I chose to use simple present "attend" rather than present continuous "are attending"
2. (most important things) Most important is only 1 thing: not to be late. So "things" should not be plural. 　
3. (being late is an impolite to others) Impolite is an adjective, therefore does not need "an". Impolite is understood to be impolite towards others so no need to say "to others"
4. (It seems like to be play trick on others) I don't really understand this, maybe you can 補充 and I'll edit.
5. (But if you late for an accident) Never start a sentence with "but" use "however"
6. (then, when you arrived) Your essay is in present tense, so should keep in present tense
7. (speak sorry) Correct should be "say sorry" and we often use "apologize" (說對不起 vs 道歉)
8. (So, being late is always be unfriendly...) You did not explain why it is impolite/unfriendly, you only explained what to do if you are late. So should choose a different conclusion, ex. Therefore, if you follow these steps, you can treat your lateness politely.

句字結構不錯，通順，加油！

When you [are going to attend] an important appointment, the most important [thing] is not to be late. Being late is impolite to [the] others. It seems like [fooling the] others [too].

But if you [are] late [because of] an accident, what can you do? First, you have to [call the] person [you are going to meet] to explain clearly [why you will be late]. Then, when you arrive, you must [apologize] to him/her. [Always] be careful of your words.

[Undoubtedly], being late is unfriendly to [the] others. To [avoid being late], [check your] timetable to find [whether or not] the [meeting] time is suitable for you. [Also], [attend] the appointment [as] early as possible. [By] following these, the [chance] of [being] late can be [reduced].

除咗少少嘅grammar嘅問題外，其他方面如 content 同 structure 都唔錯，加油啊。