Using perfect forms for ''for''

Question

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?

chasly - supports Monica · Accepted Answer

1.- What if the action is finished? For example, what if I have already left London after spending 3 years there?

You should say, "I lived in London for 3 years."

2.- If my plan is to stay in London for* 3 years.

You should say, "I will be staying in London for 3 years."

2a. - 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.

That is incorrect. The sentence, "I will have been living in London for 3 years" means that you are still living in London and that, at some specified time in the future, the duration of your stay will be 3 years.
Example
Q: How long have you lived in London?
A: Approximately 3 years. In fact, in a month's time, I will have been living in London for exactly 3 years.

Kate Bunting · Answer

We use for with a period of time, since with a starting date.
(1) I lived in London for three years if you no longer live there. The past perfect is used when speaking of a time in the past when you still lived there - In 2010, I had been living in London for three years.
(2) Yes, you can use for of a period of time in the future.

Using perfect forms for ''for''

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