英语书信开头的高级句子可以有以下几种表达方式：
表达感谢
I am writing to express my gratitude for your kind help and support.
I am writing to express my heartfelt thanks for your generous gift.
表达问候
I hope this letter finds you in good health and high spirits.
I trust this correspondence finds you well.
说明写信目的
I am writing to inform you that I have successfully completed the project you assigned me.
I am writing to request information regarding the position you advertised on your website.
表达期待
I am eagerly anticipating your response to my previous letter.
I look forward to hearing from you soon.
表达歉意
I am writing to apologize for any inconvenience caused by my mistake.
I regret to inform you that I will not be able to attend the meeting scheduled for tomorrow.
表达赞赏
I must say that I am impressed by your recent achievements.
Your work has always been an inspiration to me, and I admire your dedication.
这些句子可以根据具体情境和信件内容进行选择和调整，以确保书信的开头既得体又具有吸引力。