もし生まれ変わっても見たい映画は あなたにありますか？
When I saw the the above sentence, I started to wonder. Is it an over simplification to say that たい can only be used for your own desires? However the sentence above might be explained, it seems to me it is clearly not about the speaker's own wish, but about which movie(s) the reader would want to watch.
Under what circumstances or conditions is it permissible to use たい with a desire other than your own? Can たがる be used with one's own desire?
I'd recommend a beginner not to use たがる at all (but ～しようとする or ～したいと思う instead) because it tends to convey a contemptuous nuance and depending on cases, the meaning is slightly different from "to want", which highly matches ～したいと思っている.
Expressing other person's inner thought in indicative is avoided, in other words, it's ok if it's not indicative.
For examples, 誰か行きたい人 and あなたが行きたければ are interrogative and conditional respectively, hence, no problem in the first place. のだ forms are another way to dodge the restriction.
On the other hand, there are some exceptions.
- あなたは行きたくて、佐藤さんは行きたくないんですね。
- 私がフランスに行きたがるのは、理由があります。
Part of this are my own thoughts, part of this is taken from this paper: 中里 理子, 1992, 従属節における「たい」と「たがる」.
Often it is said that たい can be used to talk about your own desires only. While this is not wrong, it is not completely accurate either and needs some elaboration and clarification.
たい expresses a desire. た-がる literally expresses giving off the impression of having a desire. たい can be used if one does not need to assume that they know about a specific desire of a specific person, and たがる can be used if one is making assumptions on another person's wishes.
Furthermore, たい can sometimes be used for another person's desire when no strong statement is made that that person desires that object or action indeed; for example when asking a question.
Or to put it another way, it depends on the point of view. たい tends to be about a direct report of somebody's desires, たがる tends to view it from an objective or outside point of view.
「アンパン、食べたいよ」「聞きたいことがある」
「アンパン、食べたいの？」「何が言いたいんですか?」
「アンパン最高！ 世界の皆にこの絶妙な味を解ってもらいたい！ キライだなんていう人、絶対にない筈！ ね〜、花子、君も食べたい、食べたいよね？！」
「で、アンパン食べたいってどーゆーこと？」「奥さんが探洲さんのことを守りたいとおっしゃるのはよく判ります」「あなたが（田中さんが）行きたいのは、聞いています／知っています」
「アンパン食べたかったのは本当みたい」「あなたは行きたかったけれども、佐藤さんは行きたくなかったんですね」
「アンパンしか食べたくないようだが…」「友人が離婚したけど復縁したいらしい」「姉は海外留学したいそうです」「真紅はお嬢様呼ばわりされたくないようです」「もしあなたも出会いたいって気持ちが強いなら使ってみたら？」
「アンパン食べたい人、いる？」「使いたい人はいつでも自由にお使いください」(=you decide yourself if you are such a person who wants to use it)「働きたい人を募集しています」(=you decide yourself if you are such a person who wants to work)
「食べたいものある？ アンパンとかあんぱんとか餡パンとか？」「見たいところがあったら、いつでもご案内しますよ」(=you decide yourself what or which place that is, or if there is such a place)「あなたの読みたい本を貸してあげます」(=you tell me which book you desire)
Regarding the last two situations, note that it can sound unnatural if both the object desired and the person desiring it are explicit:
？ あなたの読みたい『日本沈没』を貸してあげます。
○ あなたが読みたいと言っていた『日本沈没』を貸してあげます。
○ あなたの読みたがっていた『日本沈没』を貸してあげます。
「あいつ、アンパン食べたがりそうな顔してる」「兄が結婚したがっています」
「妾がアンパンを食べたがっているのは一目瞭然。何ゆえ買ってくれんのじゃ？」「私が行きたがれば、田中さんはきっと行かせてくれるだろう」
「お姉さん以外、そんなにアンパンを食べたがる人、見たことある？」「子供はなんで荒らしたがるんですか？」「意思決定者はリスクを避けたがっているのでしょうか？」
「いろんなことができなくなってしまって、怠けてる自分やできない自分、逃げたがっている自分を責めていました」「ただ、最近の自分の生き方を見ていると、まるで『死にたがっている』ように見えることがある。」
Finally, consider some situations where たい and たがる are used.
This sentence contains both たい and たがる in a similar sub clause.
皆さん、『船に乗りたい人』はいませんか。他のクラスで『乗りたがっている人』を知っていたら、先生に教えてください。
The former one, たい, is explained by asking another person about their desires and person desiring something not mentioned explicitly. The latter one, たがる, is explained by asking someone about a third party's desires (the pupil of the class about the desires of another class).
The pupils addressed know about their own desires, thus the speaker may ask them who it is that possesses such a desire. However, the pupils addressed do not know about the desires of the pupils of another class, thus the speaker may only ask them if they know somebody who appears to possess such a desire.
In the following sentence, both たい and たがる can be used.
私がずっと行きたがっていた遊園地へやっと連れていってくれた。
私がずっと行きたかった遊園地へやっと連れていってくれた。
The former sentence is about the point of view of the person who noticed that the speaker wanted to visit the amusement park and took him there. The speaker's desire has affected the surrounding.
The latter sentence, on the other hand, focuses on the speaker's desire. The speaker always wanted to visit the amusement park, and finally, somebody took or agreed to take him there.
Here both are possible too.
あなたが（田中さんが）行きたければ、行かせてあげますよ」
あなたが（田中さんが）行きたがれば、行かせてあげますよ」
In the former sentence, the speaker directly reacts to Tanaka's wish -- Tanaka has already made it clear that he wants to go. It could also be rephrased as 行きたいというならば, if you say you want to go.
In the latter sentence, Tanaka might as well not have said it directly. That he wants is based upon the observation of the speaker, who thus takes an observational point of view and makes an assumption about Tanaka's wishes.
In the following sentence, both たい and たがる can be used.
山田君は北海道に行きたい人を知っている。
山田君は北海道に行きたがっている人を知っている。
In the latter sentence, Yamada knows somebody of whom he (or the speaker) thinks that he wants to go to Hokkaidō. It puts more emphasis on the observational point of view, it appears that that person wants to go.
In the former sentence, there is no such emphasis, Yamada simply knows somebody who wants to go to Hokkaidō. The speaker does not pretend to know anything about that 3rd person's desires, it is not the speaker who made that judgement and merely reports that Yamada is aware of such as person who might have said so.
This is definitely an over simplification, but it might be a start when one begins to learn Japanese. The usage of たい vs. たがる is rather complex and unless you are a linguist there is no need to know about all the gory details, but I believe it is helpful to think about this once and keep in mind it isn't as simple as the rule above.
