"What were you doing at 7 o'clock yesterday?" vs "What did you do at 7 o'clock yesterday?"

Question

As far as I see, both forms of question "What were you doing at 7 o'clock yesterday?" and "What did you do at 7 o'clock yesterday?" could be used to ask about someone's action in past. But are there a difference between them?

As far as I understand, when we ask "What were you doing at 7 o'clock yesterday?" we let know that we ask about some process (like "what were you doing when I've entered the room at 7 o'clock"), and if we ask "What did you do at 7 o'clock yesterday?" we let know that we ask about some momentary action (like "You did something at 7 o'clock that caused computer crash. What exactly did you do at that moment?")

Am I right or not?

Answer 1

Your sense of the meanings is right. "What did you do at 7 PM?" refers to a more or less exact time. "What were you doing at 7 PM?" means "What activity were you engaged in?". That can still mean exactly at 7 PM, but suggests that the activity extended before and after that time.
Either question could be used in your computer crash example.

Answer 2

There is actually not much difference between these questions. There is a slight implication of a single action by "What did you do" as opposed to "What were you doing", but it is very slight.

Just in case you're interested: We would also say "But is there a difference between them?" -- since you are asking about "a difference", you use the singular verb, not the plural.

Also: "we let know" is understandable but awkward. You could say "We are letting you know" or "letting the listener know"; you could even say "we are making it known"; all of these are better grammar, but a little formal for this purpose. I might suggest "We imply we are asking about some process..." or just "we are asking about a process"

One more (and ALL of these are minor): we would use an article in front of "computer crash" -- "You did something at 7 o'clock that caused a computer crash" or "... the computer crash."

I hope it is all right that I point these things out; I reasoned that it is a site dedicated to helping people with English usage, and assumed you would be interested. I admire anyone who can make themselves understood in a 2nd language, much less people who can analyze its grammar to improve their usage.