Adverb or adjective for tio

Question

• Marko: Mi volas roli kiel soldato.
• Polo: Tio estas bona / bone

Since tio is an -O word then should it be bona? Or, since the word tio is referring to a verb (roli), should it be bone?

Answer 1

It should be "bona", because it modifies/belongs to "tio", which acts like a noun. So your first guess was right. :)

In choosing whether a modifying word should be an adjective (a-word) or adverb (e-word), it only matters what kind of word it modifies. What that word means or refers to is not important.

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By the way, you can see something similar in English with "Anna is a good teacher" (Anna estas bona instruisto).

Having just learned the difference between adjectives and adverbs, English learners who don't have such a strong adjective-adverb divide in their native language are sometimes inclined to say "Anna is a well teacher" (Anna estas bone instruisto), because "good/well" is about the teaching (instrui), not about Anna herself.

But the meaning of the words are completely unimportant when it comes to basic grammar. You say "good / bona" simply because it belongs to the word "teacher / instruisto", which is a noun. That is all that's important in this case.

Answer 2

Tio isn't quite an o-word. It's a correlative and they follow slightly different rules. Still, the correlatives ending in O do have o-word-like qualities. They will take an -n ending and are described by adjectives. The biggest difference is that they don't take a plural -j.

In a sentence without tio you would use an adverb:

• Estas bone ke vi volas roli kiel soldato.