This is a sharp pocket knife.

这是一把锋利的小刀。

What do you do with my pocket knife?

你用我的小刀做什么了？

So there we are. So here is my pocket knife.

我们开始，这是折刀。

You can "do" your nails with a pocket knife.

用小刀就可以修剪指甲。

My brother bought me a pocket knife yesterday.

我的弟弟昨天给了我一把小刀。