She wrote a letter of apology but only under protest.

她无奈之下写了一封致歉信。

《牛津词典》

In 'She wrote a letter', the verb 'wrote' is transitive and the word 'letter' is the direct object.

在She wrote a letter一句中，动词wrote是及物动词，letter一词是直接宾语。

《牛津词典》

She wrote a letter afterwards.

她后来又写了一封信。

youdao

He wrote her a letter stating that he did not want her back unless she returned of her own free will.

VOA: special.2009.06.27

And,in a letter, she wrote that she was beginning to feel that the German Nazi dictator Adolf Hitler was a very great man.

VOA: special.2009.06.28

She spent her 21st birthday on board the rescue ship Carpathia, from where she wrote a letter to her mother, who had remained in Cornwall, saying she had been "thrown" into a lifeboat wearing just her nightdress and boots.

BBC: Cornwall

Natoma Canfield -- self-employed cancer survivor from Ohio -- she wrote us a letter.

WHITEHOUSE: Fighting for Health Insurance Reform

In a letter she wrote to a friend that summer, she said Tommy had hit on her, but she wasn't interested.

CNN: Moxley Case: Who was Martha Moxley?