Thora Wrote:3rd person Vたがる requires を. (I dunno why - maybe b/c the desire is less immediate/personal when you're talking about 3rd parties?)
Simple answer: otherwise 「コーヒーが飲みたがる」suggests you have unusually thirsty coffee.
The in-depth answer, exploring the mysteries of noun roles in Japanese, the difference between 消える/消す, why ~が分かる instead of ~を分る, and just what 楽しむ means begins with this: がる and たい have different theta-grids.
Yeah, real clear, right? Well, I only dabble in linguistics, but sometimes that dabbling pays off. In this case, we're looking at the relationship between the
thematic roles and
theta roles of nouns. That relationship is the theta-grid, and it's determined (primarily) by the verb.
Noun roles are the difference between "you drive car" and "car drives you." Thematic roles are the underlying meaning, theta roles are how the grammar expresses them.
For example, in English the surface roles are marked by subject and direct object position. In Japanese 「お前が車を運転する」「車がお前を運転する」the surface roles are marked by が and を.
The thematic roles are
agent and
patient. The agent carries out an action, the patient is affected by an action. Thus, "you(agent) drive car(patient)" and "car(agent) drives you(patient)." Since thematic roles express meaning, they correspond across translation--at least usually.
Now consider "I(agent/patient) look at the sky(theme)" 「僕は(agent/patient/topic)空を(theme)見上げる。」. I'm going to blithely ignore "topic," and point out two things. First, one noun phrase can take on multiple roles--this is a reflexive: looking is something you do (agent) which affects yourself (patient). Second, the "theme" role which describes "what is observed" by verbs of observation, emotion, etc. Themes aren't affected by the action. (Sorry, that's not a very good definition. Hopefully the following examples will clear it up.)
In English, themes either end up in direct object position or marked with a preposition, usually "at" or "about." In Japanese, they usually take が or を.
"Do you(patient) know him(theme)?"
「かれを(theme)知っている?」
"Do you(patient) understand the example(theme)?"
「例が(theme)分かる?」
"I(patient) like this one(theme)."
「これが(theme)好き。」
Now we can turn our attention to two areas I used to find confusing about Japanese.
First, "transitive/intransitive" verb pairs. I'm no fan of those terms, preferring "other-verbs" and "self-verbs," which incidentally are pretty close to the Japanese terminology.
Other-verbs usually have the theta-grid 「(agent)が(patient)を」「彼が木を倒した。」
Their corresponding self-verbs have the theta grid 「(patient)が」「気が倒れた。」
Sometimes, you have verb pairs where both verbs are transitive. Here's an example:
「(patient)が(theme)を知る」
「(agent)が(patient)に(theme)を知らす」
This follows the same sound pattern as the 「散る」「散らす」transitive/intransitive pair, and the same theta-grid pattern as the 「見る」「見せる」pair.
So, that's why I prefer the term "agent/patient-subject pairs". Which raises the question. Are there theme-subject verbs as well?
Yes. That's the second point I had difficulty with. Most of these verbs are adjectives--but they count because they establish theta-grids. The most common (and only?) true verb in this category is 「(theme)が(patient)に分かる」.
「(theme)が綺麗。」
「(theme)が恐ろしい。」
「(theme)が楽しい。」
「(theme)が好き。」
「(theme)が欲しい。」
Which brings us to the たい form. Like 欲しい, たい works with the theme role. 「牛乳が飲みたい」doesn't actually affect the milk, so milk doesn't take a patient role. (Exception below.)
If you need to express the patient, the がる inflection changes the theta-grid to allow that. 「猫が牛乳を飲みたがる。」The ~い adjectives with ~み noun-forms (楽しみ、愛しみ、痛み、etc) have a somewhat archaic ~む verb form with the same meaning as がる.
「犬が楽しむね。」
(This also suggests something like 「俺がこの猫に牛乳を飲みたがらせるぞ!」would bring the three thematic roles together, like English "I(agent)'ll make that cat(patient) want to drink milk(theme)," but I haven't seen it yet.)
So, that only leaves the mystery of the を~たい form. I think it boils down to language drift and the fact that language isn't always logical. 「コーヒーを飲みたい」sounds enough like 「コーヒーを飲む」that now that 「度(た)い」isn't wandering around by itself anymore, people don't notice that they're basically saying 「コーヒーを度い」which would sound wrong--い-adjectives don't pair with を-marked nouns.