Past Simple – ‘We Don’t Install’ vs ‘We Didn’t Install’

Believe or not, we do say "in 1939, a world war breaks out and lasts for six years" even though this event happened a long time ago. This is called the historical present. This grammatical phenomenon even has its own Wikipedia page. What we are doing is that we are trying to make a past situation more vivid in our mind as we speak about it. One way to achieve that is to use present-tense constructions. The page has got all the details. So, I'm not going to elaborate on it any further here. If you want to see more examples of how the historical present is used in real life, check out the Associated Press's Today in History television show. They use it a lot there.

But on a more serious note, why would you use the present tense to describe things that happened in the past even though they seem like general truths? "Seem" is the operative word here. A past event is a past event is a past event. It's, first and foremost, an event. General truths, on the other hand, are more than events. They are things that happen in general (water boils at 100 degrees Celsius, Washington, D.C. is the capital of the United States of America). An event that took place in the past stays there. WWII doesn't happen on September 1 every year. WWII is not really a general truth for that matter, it's a historical event which is a one-time occurrence. You're simply conflating two different pieces of terminology here: scientific facts and historical facts. While historical facts are in a sense as much facts as scientific ones, they're tied to a particular date on the timeline of world history which makes it inescapable to talk about them only in the past. Normally, it just semantically does not make any sense to talk about something that happened in the past in terms of the present. There's nothing else to discuss.

However, we do use present-tense verbs for general truths sometimes while talking about something that happened in the past. Both ways actually work. But this is really just an artifact of how people use and misuse tenses in general:

The Greeks actually knew that the earth is round.

The Greeks actually knew that the earth was round.

Regarding:

Have you known that Harald has a very bad disease?

I think most people would say:

Did you know that Harald has a very bad disease?

because you are asking what someone knew at the present time. Unlike seeing (where you see something at some point in time), you generally don't "unknow" some piece of information.

However, you can explicitly refer to a time progression like this:

How long have you known that Harald has a very bad disease?