Some scholars have suggested a relationship between the demonstrative pronoun 이 ‘this’ (as in: 이 사람 ‘this person’) and the following:

1. pronoun 이 ‘person’,
2. nominaliser 이,
3. particle 이(가), and,
4. the copula (이다).

Note that this theory is not commonly accepted. However, it is interesting to consider. A summary is as follows.

a)

The demonstrative pronoun ‘this’ (이) performs the function of referencing and pointing to an item in the world.

b)

It is common for languages, including Korean, to have the same word for ‘this’ and ‘this person’.

Consider:

이 책
This book

이 이
This person

c)

Nominalisers and pronouns are both devices which enable speakers to refer to entities.

d)

In 향가, functional items such as case markers were represented by Chinese characters. Particle 이(가) was represented by 是.

e)

是(si) = is/be/become in Chinese and is akin to 이다.

f)

Particle 이(가) is suggested to have originated in the development of subject as a grammaticalised topic.

Demonstration:

세본 이 상두산애 가시아.
Buddha, this went to Mt. Sandu and…  (topic)
세본이 상두산애 가시아.
Buddha went to Mt. Sandu and…  (subject)

g)

The copula (이다) is suggested to originate from pronoun/demonstrative 이, by following the complement and preceding the sentential end-marker 다. This is not a novel idea as the copula in ancient Egyptian developed from a demonstrative following a topic.

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— Rhee, S. (2008). On the rise and fall of Korean nominalizers. In M. J. López-Couso & E. Seoane (Eds.), Rethinking Grammaticalization: New perspectives (pp. 239–264).