When anyone opens a current account at a bank, he is lending the bank money, repayment of which he may demand at any time, either in cash or by drawing a cheque in favour of another person.Primarily, the banker-customer relationship is that of debtor and creditor—who is which depending on whether the customer's account is in credit or is overdrawn.

参考译文：当任何人在银行开一个活期账户时候，就等于把钱借给了银行。这笔钱他可以随时提取，提取的方式可以是取现金，也可以是开一张以他人为收款人的支票。银行与储户的关系主要是债务人和债权人的关系。究竟谁是债务人谁是债权人，要取决于/要看储户账户是有存款/结余还是透支。

Primarily, the banker-customer relationship is that of debtor and creditor—who is which depending on whether the customer's account is in credit or is overdrawn.(备注：作表语的that指代relationship,  of debtor and creditor为介词短语作后置定语修饰that.)

参考译文：银行与储户的关系主要是债务人和债权人的关系。究竟谁是债务人谁是债权人，要看储户是有结余还是透支。

以上句子语法结构分析如下

【1】the banker-customer relationship【主语】// is【系动词】// that of debtor and creditor【表语】//—who is which depending on whether the customer's account is in credit or is overdrawn【独立主格结构作状语】.

以上独立主格结构的语法结构分析如下: who is which为逻辑主语+现在分词构成独立主格结构

【2】who is which【逻辑主语】// depending on whether the customer's account is in credit or is overdrawn【逻辑谓语】//

以上逻辑谓语的语法结构分析如下：现在分词+whether 引导宾语从句 结构

【3】depending on【现在分词】// whether the customer's account is in credit or is overdrawn【宾语从句】//

以上宾语从句的语法结构分析如下：whether+主语+系动词+表语 结构

【4】whether【从属连词】// the customer's account【主语】// is【系动词】// in credit【表语(1)】// or【连词】// is overdrawn【表语从句(2)】//