Learn English – “This worked” vs “This would work”

It's a little unclear to me (native American English speaker) whether in polite sentences the modals are, formally, in the subjunctive mood, or indicative but counterfactual conditionals. (Some reference here: https://en.wikipedia.org/wiki/English_modal_verbs#Past_forms)

Either way, though, they are polite because they express a condition that is doubtful or contrary to fact. You can see this behavior in some other examples:

He can go to the store

Implies simply that he is able

He could go to the store

Implies that he is able, but in most contexts implies that he won't. When you say:

Would you go to the store with me?

You're expressing a doubt as to whether or not they will, which is a less forceful way of asking.

To me, the difference is when you had decided to end the action.

Had the action ended in the past before another action that had already ended as well, you would use past perfect (had + past participle).

Had the action ended simply in the past before the current action, then I would use past simple.

See if my explanation was clear enough for you by quizzing yourself here.