Using perfect forms for ''for''

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?

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.

• Thank you for your answer. Examples are always clarifier. What I do not understand is how, from this question, you can say I do not understand these verb tenses at all. This is a question about verb tenses when using ''for''. Typically, this is explained only for the present case, and the importance of a perfect form is emphasised. Hence my confusion. Commented Oct 26, 2020 at 13:47
• I have removed my remark. As you can see, I used "for" in my examples. Commented Oct 26, 2020 at 13:56
• You didn't have to. I wasn't saying you're wrong. Commented Oct 26, 2020 at 14:13

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.

• Thank you. Regarding (2), what I meant is if I can use it without a perfect form. I guess that something like I will have been living in London for 3 years does not make much sense in that way, since from that I deduce the action is already finished, I need some extra time reference. Anyway, I have edited my question to make explicit that remark. Thanks. Commented Oct 26, 2020 at 13:13
• This is explained in Chasly's answer. I will have been living in London for 3 years makes sense if you live there now and are speaking of a future date when the 3 years will have elapsed. Commented Oct 26, 2020 at 14:23
• My pension starts in 4 years time. If I move to London next year then by the time I retire I will have been living in London for 3 years. Perfectly natural phrasing, even though I don't currently live in London. But I can't see any good reason to use the continuous verb form there, as opposed to ...I will have lived in London for 3 years. Commented Oct 26, 2020 at 14:50