将两个句子改写成一个定语从句时，应遵循以下步骤和顺序：

确定先行词：

首先，需要确定哪个名词或代词将被定语从句修饰。这个名词或代词称为先行词。

选择关系词：

然后，选择一个合适的关系词（关系代词或关系副词）来引导定语从句。关系代词在从句中充当成分，而关系副词在从句中充当状语。

构造定语从句：

将另一个句子的内容改写成定语从句，使其修饰先行词。通常，定语从句会跟在先行词后面。

合并句子：

将主句和定语从句合并，确保语法正确且意思清晰。

示例

原句

1. The food tasted bad.

2. We had it in the restaurant on the corner.

改写后的定语从句

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

在这个例子中，“the food”是先行词，定语从句是“that we had in the restaurant on the corner”，关系词是“that”。

原句

1. The knife is very sharp.

2. I use it to cut many things.

改写后的定语从句

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

在这个例子中，“the knife”是先行词，定语从句是“which is very sharp”，关系词是“which”。

原句

1. My younger brother is a lovely boy.

2. I often buy some small gifts for him.

改写后的定语从句

1. I often buy some small gifts for the boy who is my younger brother.

在这个例子中，“the boy”是先行词，定语从句是“who is my younger brother”，关系词是“who”。

总结

通过以上步骤，可以将两个句子合并为一个定语从句，使句子结构更加紧凑和清晰。关键在于确定先行词，选择合适的关系词，并正确构造定语从句。