后天

Chinese time words sit in a slot right in front of the verb. English dumps time at the end, so learners write 我去北京后天. Flip it and you sound native: 我后天去北京. If the whole sentence is about that day, 后天 jumps to the very front. 后天有个好电影 and 后天有个会 both work. Questions follow the same slot: 你后天有空吗, never 你有空后天吗.

“后天有个好电影。” (The day after tomorrow, there's a good movie.)

在 mostly pairs with places: 在家, 在公司, 在健身房. With bare time words like 今天, 明天, 后天, Chinese drops it. So 后天见, 后天交房租, 后天再说 all stand on their own. The 在 returns the moment 后天 glues onto a noun: 在后天的面试上. There it's describing a specific event, not marking a time slot.

两天后 just means "two days later" and counts from whatever moment you just mentioned. Say 他周五出发，两天后到 and you land on Sunday, not on a fixed date. 后天 needs no starting point, because everyone knows the clock starts today. You also can't stack it: there's no 三个后天, but 大后天 covers three days out.

先天 is what you're born with; 后天 is what life adds afterwards. 先天条件 talks about natural gifts, while 后天努力 is the work you put in later. You'll meet the pair in articles about talent, health, and habits. Context decides everything, so 后天见 is a date and 后天培养 is nurture. In a biology or upbringing text, read it as "acquired."