前 (qián) and 后 (hòu) are interesting because their meanings change depending on whether you're talking about position or time.
For position, you have:
前面 qiánmiàn in front / the front
后面 hòumiàn behind / the back
For example:
我家前面 wǒ jiā qiánmiàn in front of my home
我家后面 wǒ jiā hòumiàn behind my home
But when talking about time:
三天前 sān tiān qián three days ago
三天后 sān tiān hòu three days later / in three days
This was confusing to me because I had already associated 前 with front and 后 with back.
My instinct was to think of the future as being in front of me and the past as being behind me. Following that mental picture, I kept wanting 前 to point toward the future and 后 to point toward the past.
Instead:
前 → past 后 → future
This ended up being the one question on my HSK1 mock exam that I fully accept getting wrong. I knew I needed to express "three days later," but I confused myself over whether that meant 三天前 or 三天后.
A timeline makes the relationship a little easier to see:
← Past
三天前 sān tiān qián three days ago
前天 qiántiān the day before yesterday
昨天 zuótiān yesterday
今天 jīntiān today
明天 míngtiān tomorrow
后天 hòutiān the day after tomorrow
三天后 sān tiān hòu three days later / in three days
Future →
The part I have to remember isn't really the vocabulary itself. It's that the spatial association I naturally make in English doesn't match how 前 and 后 map onto time in Chinese:
前面 → in front 后面 → behind
but
前 → earlier / past 后 → later / future