in advance 时间上 预先.

"when you pay in advance you receive a discount"

beforehand 除了有时间上 预先，超前外，还有非时间上 的，例如 “需要”上 预先，超前.另外有过早的意思.

She is always beforehand with the rent.她总是早早把租金准备好.

We were aware of the problem beforehand.我们事先就知道有这个问题.