All.You can apply this in both Thai

All.

You can apply this in both Thai and English writing.
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Letters of complaint
Most people will need to complain about unsatisfactory goods or bad service at some point in their lives. If you are writing a letter of complaint, there are some dos and don’ts that can help you get the right outcome.
Do
gather all your facts before you start writing the letter, e.g. where and when goods were bought, any customer reference numbers or invoice numbers, or a record of any previous communication with the company concerned.
get straight to the point and set out exactly what the problem is.
give all the relevant information in a clear and logical sequence.
state what action you want the company to take and when you expect them to have done this.
enclose copies of any relevant documents that support your case.
Don’t
use abusive language, however frustrated or angry you might feel.
allow yourself to get sidetracked from your specific problem into generalized criticism of the company or its products or services.
send your letter without checking it carefully for grammar or spelling mistakes.
Structuring your letter
The opening
Start your letter with the greeting Dear Mr (or Mrs, Ms, Miss, etc.) Surname.
If you don’t know the name of the person you are writing to, begin with Dear Sir or Dear Madam; if you don’t know their name or sex, use Dear Sir or Madam.
The content
Begin with a heading alerting the reader to the subject of the letter, and in your first sentence draw their attention to the matter you’re going to raise or discuss. For example I am writing to complain..., or I wish to express my dissatisfaction with ... or Thank you for your letter of ...
Introduce your main point as early as possible, stating your reason for writing in a clear, concise way. Once you have done this, you may want to give more details, perhaps adding further background or relevant facts.
In conclusion, you should state what your expectations are, for example Please let me know as soon as possible what action you propose to take or I look forward to hearing from you within the next ten days.
The close
The wording at the end of a formal letter follows a standard format:
If you know the name of the person you’re writing to, you should end the letter with Yours sincerely.
If your letter begins with Dear Sir or Dear Madam, it should end with Yours faithfully.
Your own name should be typed out underneath your signature.