I came across a sentence using definite article 'the' with comparative degree and an other sentence with out "the".
Can anyone explain when to use 'the' with comparative degree?
Here are the sentences:
[1] Which one of the twins is the smarter?
[2] Which of these two books did you enjoy more?
In [1] "the" is determiner in the fused modifier noun phrase "the smarter", which in full means "the smarter one". Note that count nouns like "twin" require a determiner.
In [2] "more" modifies the verb "enjoy". It is not part of a noun phrase like "smarter" is in [1].
"The" is the determiner for the definite article.
"These" is the plural of the pronoun this.
In your two examples, you have two plural nouns - "books" and "twins", and as you show, either can be used. It has nothing to do with the comparative degree.
When you use the definite article "the", it has to be clear who/what you are referring to. This would normally be established in the context - for example, you might have named a particular set of twins in your preceding sentence, in which case it is clear who "the twins" are.
When you use "this" or the plural "these", the specific person or thing you are referring to is normally close at hand, being indicated, or currently being experienced. For example, if you took someone into an art gallery and asked "which of these paintings do you prefer", it would be clear you were referring to the paintings right in front of you, and you would not necessarily have to have named them verbally first.
So, either of your examples could be correct in different circumstances or contexts.
