With reference to the above my question, I think I came to know the reason why a part of speech of the word 'only' in the sentence "Only God can make a tree." is not an adjective but an adverb.
The reason is that the word of only in the sentence has no descriptive element about God. That is, No word can be an adjective without having a descriptive meaning. The word of only in the sentence only restrict or limit the word of God with the meaning of " no one else but ." Therefore, it can not be an adjective but an adverb.
I would like to inform you of the fact that I owed much to a Korean linguist who helped me to understand the strict meaning of an adjective.
Sariputra from Korea
Thank you for taking the time to send us the added information, Sariputra. ^0^ It's much appreciated.
All the best,