44．Rent control is the system whereby the local government tellsbuilding owners how much they can charge their tenants in rent.

(whereby=by which，引导定语从句，表示手段)

租金管制是政府借以告诉房主他们可以收取多少租金的系统。

分句1：Rent control is the system

分句2：whereby the local government tellsbuilding owners

分句3：how much they can charge theirtenants in rent.

分句1是整个长句子的主句

分句2是分句1的定语从句，whereby相当于我们熟悉的by which

分句3是2的宾语从句，修饰tell 的内容宾语（因为tell有双宾语）

1和2嵌套

2和3嵌套