1. Could I borrow a pen and three pieces of paper?

能借我一支笔和3张纸吗？

还能这样说：

Can you give me a pen and three pieces of paper?

Would you lend me a pen and three pieces of paper?

应用解析：

a piece of cake 轻松愉快的事情；

break into pieces 使成为碎片；

by the piece 按件计算

come to piece 破碎，碎裂；身体（精神）垮下来；崩溃

2. I want to write a letter to my pen friend.

我想给我的笔友写封信。

还能这样说：

I want to write to my pen friend.

I would like to write a letter to my pen friend.

谚语：

Never write what you dare not sign.

不要写你不敢署名的文章。

3. My dear mother. I am missing you very much.

亲爱的妈妈，我非常想念你。

还能这样说：

Dear mom, I miss you so much.

To my great mom, I miss you terribly.

谚语：

Poverty is the mother of all arts.

穷后而工乃是艺术之道。

4. She writes to her friend once a week.

她一星期给她的朋友写一封信。

还能这样说：

Every week, she writes to her friend.

She writes a letter to her friend once a week.

应用解析：

all at once 突然；同时；

at once 立刻，马上；同时；

every once in a while 偶尔；

once too often 又一次