A letter to my mother

I will write a letter to my mother.

我要写一封信给我母亲。

My mother has sent me a registered letter, I guess she has something important to tell me.

我妈给我发了一封挂号信，我想她有什么重要的事情要说。

He wrote a letter for my mother saying that it was likely he wouldn't return, because the flight was extremely dangerous, and that he wanted her not to remain on her own in that case.

父亲给母亲写了一封信，他在信中说，此次飞行任务极端危险，自己很可能回不来了，他希望母亲能够改嫁。