owing money that should have been paid already

My account is badly in arrears. 我账户上的欠款太多了。

They are in arrears on/with their mortgage payments. 他们的按揭贷款逾期未还。

If someone is paid in arrears, they are paid at the end of the period of time during which the money was earned.

I'm paid a week in arrears. 我每周末领薪水。