把两个句子改写成一个定语从句。顺序是什么？

这三个句子都可以改写为定语从句。

1. The food that we had in the restaurant on the corner tasted bad.

  2. I use the knife which is very sharp to cut many things.

  3. I often buy some small gifts for my younger brother who is a lovely boy.

改写定语从句的时候首先要先确定被修饰词是那个，也就是我们说的先行词，之后确定关系代词或是关系副词。初中阶段常用的关系代词有who, whom, whose, which 和that， 常用的关系副词有when, where 和why. who, whom whose 和that指代人，which，whose和that指代物。

The food that we had in the restaurant on the corner tasted bad.
我在街角餐馆吃的食物很难吃。原句分别是食物很难吃；我们在街角餐馆吃食物。两句重复的是食物，可以以food为先行词，加that or which 链接两个句子。下面的同理
The knife which I use to cut many things is sharp.
My younger brother for who I often buy some small gifts is a lovely boy