This is the 4386th most frequent Chinese word.
感谢信
Thank-you letter.
The word '感谢信' refers to a written letter specifically used to convey thanks in this sentence.
我写了一封感谢信送给我的老师，以表达我的感激之情。
I wrote a thank-you letter to my teacher to express my gratitude.
Here, '感谢信' is discussed in a general sense as a tool or method for expressing gratitude formally.
感谢信可以帮助我们更正式地表达我们对他人的感谢。
Thank-you letters can help us express our gratitude to others more formally.
In this sentence, '感谢信' is directly referring to a specific thank-you letter received or sent.