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In the first year alone of my learning Korean I collected the following clearly related but also unclearly related words:
그래서 – so
그래요 – that’s right; OK
그러면 – in that case
그러니까 – therefore
그러지요 – OK, I’ll do so
그런데 – by the way
그럼 – then
그리고 – and
그렇지만 – however
그렇다 – to be so
I tried to look up these words in Jungbum Seo’s “Korean Etymological Dictionary”, only to find that it mentions none of them (out of 1500 words). But it turns out that by simply using Naver one can already create considerable order in the above chaos.
It is reasonable to assume that all these words derive from or at least are connected to the verb “그렇다–to be so”, which can be found in any elementary text book. Naver gives us its original form, 그러하다; its stem is 그러하-, which over time reduced to 그랗-, which yields the present-day verb 그렇다.
This stem is used before some endings, for example –다, and –지만“but”. This immediately gives us
그렇지만 ← 그렇-+-지만 (stem + –지만) = that is so but → however,
where the ← means “is derived from”, and → means “leads in meaning to”.
Other endings use the infinitive, the form that is usually obtained by adding –아-/-어– to the stem, as in 먹어요 from 먹다. In this case we start with the old stem 그러하-. All verbs in 하– change their 하– to 해– in the infinitive, so 그러하– yields 그러해-. Over time this infinitive reduced to 그래-, the present-day infinitive. This 그래– gives us
그래요 ← 그래-+-요 (inf. + –요“polite”) = that is so → that’s right; OK
그래서 ← 그래-+-서 (inf. + –서“because”) = because that is so → so.
The endings –으면 and –으니까 follow the stem, and keep the 으 when the stem ends in a consonant:
그러면 ← 그렇-+-으면 (stem + –으면“if”) = if that is so → in that case,
with reduction of 렇으 to 러; or with even more reduction, both in form and in meaning:
그럼 ← 그럼–연 ← 그러면 = in that case → then.
The same reduction of 렇으 to 러applies to the ending –으니까:
그러니까 ← 그렇-+-으니까 (stem + –으니까“because I know”) =
because I know it is so → therefore.
The stem-following ending –지 has several meanings; one of them (in affirmative sentences) is that of reassuring the listener, for example: 했지요– I assure you, I’ve done it. This meaning is found in
그러지요 ← 그렇-+-지-+-요 (stem + –지– “reassuring” + –요“polite”) =
I assure you that it is so → OK, I’ll do so,
with loss of the ㅎ. Remarkably, Naver shows that the most usual meaning of –지, “asking for confirmation”, also occurs in the same construction:
그렇지요? ← 그렇-+-지-+-요? (stem + –지– “seeking confirmation” + –요“polite”) =
don’t you agree that that is so? → isn’t it?
but now without the loss of the ㅎ.
The old form of the stem is required to explain 그런데:
그런데 ← 그러하-+-는데 (stem + –는데 “against the background of”) =
against the background that that is so → that being so → by the way,
with reduction of –하는– to a single ㄴ.
These explanations find support in the fact that some forms exist in which 그– is replaced by 이– or 저-:
이래서 – for this reason
저래요 – it just is like that,
but none of these has as wide a collection of forms as 그-.
This leaves the form 그리고–“and”. The ending –고 means “and” after the stem of a verb but no applicable verb stem 그리– seems available, so for a while I puzzled over it. But Sohn, “The Korean Language” (1999), pg. 261, has the answer: 그리고 derives from the old noun 리 – direction, which is still used in “이리 앉으세요” – “please take this seat”. So we have
그리고 ← 그 리 하고 = this/that way doing and → and,
and 그리고 is not related to 그렇다 (unless of course if 리 and 그렇다 are related, but that is lost in the mists of time).