It depends on your point of reference. You could say "eight months from now" or "in eight months" I will be finished university. "During the next eight months" I will be studying very hard. "Eight months ago", I moved to Europe. For the past eight months I have lived in Europe. When you use "for the past eight months" or "during the next eight months" then you are making reference to a past or upcoming continuous length of time. You can do it with days, weeks ,and years. Examples: "Two days from now" or "in two days" ; "for the past two days" ; two days ago" It's the same with weeks and years. Good luck with your learning goals!

The phrases you are asking about all have slightly different meanings. "8 months from now" -- on a date 8 months from today. Example: I'm moving to Atlanta 8 months from now. "in 8 months" -- means exactly the same thing as "8 months from now." Example: I'm moving to Atlanta in 8 months. "the next 8 months" -- starting today, ending on a date 8 months from today. Example: I'm going to live in Brooklyn for the next 8 months. "8 months until ___" -- means there are 8 months until something happens. Example: Only 8 months until I graduate college! Colloquially, people generally say "8 months till ___" (not the full word "until") "8 months ago" -- opposite of "8 months from now." specifies a date 8 months in the past. Example: 8 months ago, I had my first child. "the last/past 8 months" -- opposite of "the next 8 months." specifies a time period that started 8 months ago and ends today. Example: For the last 8 months I haven't spoken to my brother. Hope that helps!