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Hi teachers,

His responsibility is the greatest of all.

Here ‘the greatest’ is an adjective because it is after the verb to be, isn’t it?

What is ‘of all’ in this sentence? Is it a pronoun?

Thanks in advance
Comments  
All is a pronoun, and greatest can be considered either an adjective or a pronoun, too.
His responsibility is the greatest of all. Here it is a pronoun 'greatest', right?

His is the greatest responsability of all. Here it is an adjective, right?

This is the only way, at least for me, to understand that the 'greatest' can be both: an adjective and a pronoun.
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Mister Micawber, is it as I've said or I'm completely wrong? Thank you.
This is the only way, at least for me, to understand that the 'greatest' can be both: an adjective and a pronoun.
How can I know whether that is the only way for you?

Well, there are only two ways as you've said: adjective and pronoun. How come a word can be two grammatically options at the same time in a sentence? That's where my doubt is. Thanks.
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I think it's a matter of how the grammarian views it:

Marmaduke the magician is the greatest (magician).

As you can see by the parentheses, the noun is optional. Without it, the function of 'greatest' is also optional. That is how I see it.
Thank you Mister Micawber and now, after your commentary, it is the way I understand it.