Is "only" an adverb or adjective?

ex. I only know one thing.

I know only one thing.

The word only is an adverb in your example.

It's the only tree.

It's an adjective that example though.

So, it can be either. It depends how you use it.

is only an adverb or an adjective in this sentence: Mary and Mike are the only children of Pete and Rose.