Annuity Due:
An annuity-due is an annuity whose payments are made at the beginning of each period.
Example, deposits in savings, rent or lease payments, and insurance premiums are of annuities due.

Annuity-immediate (ordinary annuity):
If the payments are made at the end of the time periods, so that interest is accumulated before the payment, the annuity is called an annuity-immediate, or ordinary annuity.

Annuity-certain:
If the number of payments is known in advance, the annuity is an annuity-certain.

通常他会告诉你payment的时间点，一般什么都不说按annuity-imm处理。

一般考试due的题目会多些，因为imm可以直接套用公式，而due虽然有公式，但是第一项总是不乘以系数比如利率等，容易出现错误。楼上说很对，一般如果是insurance相关题目，premium经常在年头给付，而benefit通常在年末。

不说就是imm，但题目可能会有陷阱。给你pmt的话一般两种方法都可以算到答案。

好嘞，多谢大家啦！