TESL - does anyone know of a rule for by/until?

by + time = completed before that time

until + time = ongoing from now up till that time.

I am working until 6:00 today = I am working continually, and I will stop work at 6:00.

I will start work by 9:00 today = The "start" will be at some moment before 9:00.

Now to confuse the issue a little. You can say "you must write this by 6:00". But another way to say that is "you have until 6:00 to write this". In the first sentence, the action of writing must be done by 6:00 or before. It is a definite, completed action. In the second sentence, "until 6:00" describes the continuous time period from now to 6:00, and says this is an object that you have.

Edit: I'll do it by Monday = the action will be completed on Monday or before.

He'll be back by Friday = his return will be completed on Friday or before

I have until Monday = just like my example above, "[from now] until Monday" describes a time period that you have.

I couldn't do it until yesterday = This is more difficult. It says that in the time period ending with yesterday, I couldn't do it. It's sort of like the previous, it means "from when it was assigned until yesterday". But it also means that I was able to do it yesterday, after that time period was over.

I couldn't do it by yesterday = I was not able to complete it before yesterday. There is no information conveyed here as to whether it has been done yet.

I shall do it by Monday, Until then, you will have to wait

He will be back by Friday: his return flight will take him until then.

I have until Monday to do it. But I am sure I shall have completed it by then.

I could not do it until yesterday. By which time my boss had become really annoyed with me.

"by" indicates the date on or before which something gets done.

"until" refers to the period of time preceeding some particular point (in time, or space).

I see that my English-German dictionary offers "bis" as a translation for both words, but I would have thought "bis" was closer to "until" and that "vor" would be closer to "by."