Use of the past tense in a present or future action is for subjunctive or optative moods of speech.
The third conditional for "if I could".
- Tomorrow, if I asked you to help me, would you do it?
- I would like to let you know that your services are no longer needed.
- I could go to the beach with you.
- Would you like to kiss me?
The subjunctive could be used for
- Possible but unlikely
- Retrospective (postmortem what-if analysis)
The optative is similar in use to the subjunctive to denote
- choice and the consequence if such a choice is taken.
The most well-known illustration that has been used for decades to teach subjunctive speech is:
If I were a bird I could sing all day.
The sentence meant to say,
It is impossible for me to be a bird. But what if, I were a bird.
It is not acceptable to say, due to the subjunctive situation,
If I am a bird, I will sing all day.
Imagine visiting someone's home while there is a huge snow storm and your boots are all mucky. Your host may say,
Can you take your boots off in the mud room before coming into the living room? If you take your boots off, that will make my day.
However, we normally say,
Could you take your boots off in the mud room before coming into the living room? If you took your boots off, that would make my day.
This optative mood mimics the impossiblity of taking your boots off. Your host is expressing politeness by implying - I realise it could be unlikely to take your boots off, or that it is very inconvenient to take your boots off. But if you did it, my having a happy day would no longer be unlikely.
Therefore, using the optative is a form of politeness in English speaking cultures.
An example of contingency is
If the plane went down, the oxygen mask would automatically drop down and you could pull out the life vest from under your seat.
Using subjunctive speech in such a situation is so much more reassuring to passengers than the flight steward announcing assertively
If the plane goes down, the oxygen mask will automatically drop down and you can pull out the life vest from under your seat.
As a flight steward, you would wish to express that it is highly unlikely that the plane would go down, but just in case.
Another example -
Sister (deterministic assertion): Tomorrow is a holiday. What will you be doing to spend your day?
Brother (optatively): I don't know. I could go to the beach.
Sister (contingency, in case you are going): I would like to join you.
Sister (politeness): Could I?
Brother: Of course.
Sister: Thank you.
Brother (unlikelihood): What could we possibly do to persuade dad to come with us to the beach?
Sister (unlikelihood): Would he know the way to the beach?
Brother (uncertainty): He might know how to use Google Maps.
Sister (impossibility): If he had a magic carpet, he should use it!