引入主题：

With reference to …

Further to …

With regard to …

I am writing in connection with …

注：引入主题的句子可以被写在信件的开头，但with regard to 不能用于信件的开头。

环境：(How would you start a letter about each of the following?)

an invoice (No. 679) for a photocopier

With reference to Invoice No. 679, we regret to tell you that the product arrived in bad condition.

a meeting you had with the addressee on Jan. 16th

Further to the meeting on 16 January, I am delighted to tell you that we are now able to give you a special offer for our Peach Series computer.

an advertisement in the times newspaper

I am writing in connection with the advertisement in the times newspaper yesterday.

an application for a post as secretary in your company

I am writing in connection with your application for a post as secretary in our company.

a fax order for six computers that you received today

with reference to your fax order for six computer, I regret to tell you that they are out of stock at present.