you may or may not have figured it out by now but here's a way to diffrentiate the two above and others as well
you need to know if the い is used to pronounce as it self or to elongate the previous vowel example
in "面白い(omoshiroi)" the い is being pronounced as itself which makes it a -i adjective and therfore can't use the declarative
but in きれい it is being used to elongate the "e" vowel making it a -na adjective and therefore you can add the declarative
go back to "The Writing System" page and go to the hiragana section and then scroll down to the "Long Vowel Sound" section
there you will find that the "い" actually elongates "e" vowel pronounciation hope this helps you differentiate
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you may or may not have
you may or may not have figured it out by now but here's a way to diffrentiate the two above and others as well
you need to know if the い is used to pronounce as it self or to elongate the previous vowel
example
in "面白い(omoshiroi)" the い is being pronounced as itself which makes it a
-i adjective and therfore can't use the declarative
but in きれい it is being used to elongate the "e" vowel making it a -na adjective and therefore you can add the declarative
go back to "The Writing System" page and go to the hiragana section and then scroll down to the "Long Vowel Sound" section
there you will find that the "い" actually elongates "e" vowel pronounciation
hope this helps you differentiate