What's the rule behind the use of the articles to refer to something in general? [closed]

You are confused, but that is not surprising: this is not an easy area.

First of all "man" is a special case (I will come back to this).

In normal speech, for general statements we normally use the plural without "the":

Lions are carnivores.

Computers need power.

Icebergs float.

There is a rather old-fashioned use of the singular with "the" to mean "a typical one", so

The lion has claws, a mane and a tail.

You will find this in old books, especially about animals and about nationalities and "races" of people, but it's not often used today.

You will find a similar use where you are not referring to a particular individual, but to a generalised individual which represents the entire collection of such items, or perhaps the institution, or idea, or effect of all the items:

The motor car changed the way we lived.

The mosquito has killed more people than any other cause. (I'm not saying that this example is true - it's just to show the construction!)

This construction is still in use, but it is more common in writing than in speech.

I said that "man" is a special case: it was used like "the lion" above, but without the article:

Man is a tool user.

Man has occupied every part of the Earth.

(The man is a tool user is grammatical, but cannot have this meaning: it is the usual meaning of the, referring to a particular man).

This use is much less common now, as most writers avoid using man to mean human being because it might be misinterpreted as male human being.