I wondered about the part-of-speech for the word "Yes", since I wanted my program to produce a grammatically correct response to a simple yes-no-question.

I started out by entering a yes-response sentence in the online parser of Stanford University:

Yes, my dog also likes eating sausage.

A funny sentence? Agreed, but it is the parser's example sentence, extended with the affirmative "Yes". The parser tells me that "Yes" is a "UH" or "interjection".

Promising. I had never heard of this part-of-speech. So I looked it up in the Wikipedia:

"In grammar, an interjection or exclamation is a word used to express an emotion or sentiment on the part of the speaker."
"It also can be a reply to a question or statement."

The word "Yes" was not mentioned on the page, but there is a list of interjections. This led me to this page:

http://en.wiktionary.org/wiki/yes#English

I must say I am really impressed by the time people spend extending Wikipedia and the Wiktionary.

The page says that "Yes" has four parts-of-speech:

• Adverb: as in "Yes, you are correct"
• Interjection: as in "Yes!" (you can feel the emotion)
• Noun: "Was that a yes?" (this one made me smile)
• Verb: "Did he yes that veto?"

Hmm, appararently the "Yes" I needed was more like the adverb. But wasn't an adverb a word that modifies a verb? Apparently I wasn't the only one who asked himself this question. In the page's discussion RobbieG asks:

"What's more, I don't believe the word "yes" can be used to modify a verb. Why is it listed as an adverb?"

A certain Stephen answers:

"Adverbs can modify not only verbs, but also adjectives, other adverbs, phrases, even whole sentences.
Adverbs in general answer how?, when?, where?, to what extent?, whether.
Moreover, this grammatical term in English is a catchall category where we put words that don’t fit in the more ruley parts of speech."

So I looked it up under the entry for adverb

"An adverb is a part of speech that changes the meaning of verbs or any part of speech other than nouns (modifiers of nouns are primarily adjectives and determiners). Adverbs can modify verbs, adjectives (including numbers), clauses, sentences, and other adverbs."

Interesting information. But it tells me that for something to be an adverb, at least it needs to modify something. And "Yes" doesn't. It just affirms the question.

So I thought maybe "Yes" should not be in the "catch-all" category, but a new category should be created for it. What words would be in that category?

yes, no, affirmatively, negatively, absolutely, certainly

This made me see that "Yes" doesn't mean "It's true", but rather "I agree". It means: "Your answer is according to my view of the world". Seen this way, the word does modify the sentence. It adds a stamp of approval.

So it's an adverb.

And this is confirmed by this article that even specifies it as an affirmative adverb.

But some time after I wrote this I found another Wikipedia article that explicitly opposes this view. It says that it not an adverb, nor an interjection. It is a sentence word. However, this can only be the case if the word is used by itself in a sentence. Actually I find this point of view less than satisfactory, because it would mean that the part-of-speech of the word is different when it is used as part of a sentence and when it is used as a complete sentence.