Another form of "Again".. and the day after tomorrow

又, 后, 前

Another form of "again" is 又(you).

The difference between this and 再 is that 又 is used for something in the past.

So, 我又去北京了 means "I _went_ to Beijing again".

后天 == (hou tian) == the day after tomorrow
前天 == （qiantian) == the day before yesterday

the day after the day after tomorrow? 大后天。
the day before the day before yesterday? 大前天！