My friend and I were having a casual conversation in the middle of the night. Eventually, we came up with the question - why is building still called building, even though it's already built?
There's more on the list - such as, painting, writing, etc...
I'm guessing it has something to do with gerunds, is that right? if not, why?
It is historically connected with gerunds, but not usefully in the current language.
Every verb can form a gerund meaning the action or process of ..., but only in some verbs has that meaning been extended to the physical objects or materials resulting from or used in the process; and the particular extension of the meaning varies from verb to verb.
So we can talk about a building, a painting, a drawing, a recording, some writing, some knitting, as the object produced, but we don't normally talk about sculpting, playing, creating in that sense.
We talk about shopping, sewing, and washing, referring to the things that we buy, sew, or wash; but we don't use "buying" or "cleaning" in that sense. Oddly we can talk about "mending" as a physical object in the now rather rare sense of clothes that get repaired: we can mend broken furniture or equipment, but we wouldn't refer to the piece of furniture as "a mending" or "some mending".
We can talk about the binding of a book, the wrapping of a parcel (more often "wrapping paper" where it may be interpreted as a participle, but sometimes used on its own) and bandaging and fastenings as physical things; but not usually "attaching" or "supportings".
The answer is -ing had a different role in Old English (where it was sometimes -ung). It was still used to turn verbs into nouns, but they were a different kind of noun that often referred to the product or result of the action rather than the action itself. Even when it didn't, it produced a countable noun referring to a specific act.
For example, an ascung (asking) was a question (thing that one asks), a ceosung (choosing) was a choice, a brecung (breaking) was a fracture, and a feorting was a fart. The word bylding is not attested in this period, but may well have existed with this meaning.
Since it already produced nouns from verbs, the suffix -ing started to be used to form what we now call a gerund sometime in Middle English; until then, infinitives had been used instead. This change was probably cemented when the infinitive ending -en was dropped from verbs in the transition to Early Modern English, so for instance "swimmen is good exercise" became "swimming is good exercise".
This -ing form also came to merge with the -ende verb ending that formerly marked the present participle. The process might have been something like Old English "ic eom swimmende" > Middle English "ic am swimminde" > "ic am swimminge" > "I am swimming"—thus leading to the present oddity where -ing typically has present meaning, but in words like building appears to refer to a past action.
Gerunds in English are like nouns in that they don't have different tenses like the verbs from which they are derived. I appreciate this is different in other languages such as Japanese, where there are different forms of gerund depending on the tense.
