判断一个英语句子中谁是主句谁是从句，可以通过以下方法：

从句的类型

定语从句：从句修饰名词或代词，去掉从句后，主句依然完整。例如：The book which is on the table is mine.

名词性从句：从句在复合句中充当主语、宾语、表语或同位语，去掉从句后，主句将缺少相应的成分。例如：

主语从句：That he will come is certain.

宾语从句：I don't know if she is coming or not.

表语从句：The problem is how to solve it.

同位语从句：The city of New York is known for its diversity.

句子结构

主句：通常包含主语和谓语，可以独立存在。例如：She is a teacher.

从句：依附在主句上，不能独立存在。例如：because it was raining, we stayed at home.

连接词

主语从句：通常由连词如that, whether, if, who, whom, whose, what等引导。例如：That he will come is certain.

宾语从句：通常由连词如that, if, whether, who, whom, whose, what等引导。例如：I don't know if she is coming or not.

定语从句：通常由关系词如who, whom, whose, which, that等引导。例如：The book which is on the table is mine.

表语从句：通常由连词如that, whether, if, who, whom, whose, what等引导。例如：The problem is how to solve it.

通过以上方法，可以较为准确地判断出英语句子中的主句和从句。