So, the rule I have been taught is that 'for' and 'since' must be used with perfect forms, i.e. I have been living in London for 3 years/since 2017. This presents two problems for me:
1.- What if the action is finished? For example, what if I have already left London after spending 3 years there? Should I use past perfect? I had been living in London for 3 years.
2.- What about future? For example, if my plan is to stay in London for* 3 years. Do/should I use a perfect form? In that case I think I perfect form might not be correct. For example, something like I will have been living in London for 3 years means, in my opinion, that at some point I stayed in London for 3 years AND that I moved, so I am not there anymore. In that sense, my previous sentence is missing that extra time reference, is not it?
Bonus track. I met a native last week and he told me people do not use present forms so often. Maybe in present yes, but for a past action we would use some simple past or 'used to'. What about this? Is it something that can be used but is not correct strictly speaking?
*Here it is a use of for without perfect forms. Is it correct?