Please explain how to use "tagaru" in a sentence. I understand ~tai, and from what I vaguely know, ~tagaru is used when referring to what another person may want. But I have never used it in a sentence, and I'm missing the Genki I book where it's mentioned.

Maybe explain ~hoshigaru too. I imagine it's used for what you think someone may want, as in an object the way hoshii is used for yourself. So hoshii is used for wanting an object, and tai is for wanting an action (for lack of a better word)?

The reason of using ~garu is that it is easy for you to personally know if you like or want something, because you can read your own mind. As you cannot read other people's minds, it is therefore impolite to make an statement about what they like, because you are putting words in their mouth. To avoid that, you add ~garu to the verb to specify that you "think that they like/enjoy/whatever", leaving room for them to confirm or deny it.

Cyborg Ninja Wrote:Please explain how to use "tagaru" in a sentence. I understand ~tai, and from what I vaguely know, ~tagaru is used when referring to what another person may want. But I have never used it in a sentence, and I'm missing the Genki I book where it's mentioned.

Maybe explain ~hoshigaru too. I imagine it's used for what you think someone may want, as in an object the way hoshii is used for yourself. So hoshii is used for wanting an object, and tai is for wanting an action (for lack of a better word)?

When expressing the feelings of another person, you generally don't just state them directly (this is not limited to -tai, but applies to some normal adjectives like samui, kowai, etc.). You have to add something to the feeling to indicate how you are getting this information about their feelings.

-garu is one possible way to do this. -garu means that the person is showing the emotion. So 食べたがる would mean that the person is showing that they want to eat.

If they're not showing it, other possibilities are 食べたいでしょう (maybe he wants to eat), 食べたいと言っていた (He said he wants to eat), 食べたいと思う (I think he wants to eat), etc.

EDIT:
There are some exceptions to the "-tai only with your own wants" guideline.

1. If you are asking another person about their wants, you can use たい directly.

2. If you are using a conditional or general statement, you can use たい directly. For instance, you could say 鈴木さんが帰りたいなら、帰させていいと思う。 or 帰りたい人は帰ってもいい。

(I wonder why they teach ほしがる・たがる to a beginner, because it's difficult to master.)

It has two meanings:
(1) to act as if you want something (similar to ～しようとする:to try to do)
(2) (a kind of rhetoric expression of) to want (similar to ～したいと思う)

Before the main question, let's confirm that, as is mentioned in other member's post, you basically have to avoid expressing other person's inner thought IN INDICATIVE. (in other words, it's no problem in the first place as long as the sentence is not indicative like request, question, conditional, のだ form or kinds of quote)

Then, let's consider it through comparison with ほしい.

水がほしい人 someone who wants water
水を欲しがってる人 someone who asks asks for water (meaning1)

Those won't be a problem, right? Then, what about these?

i. (now here) 水がほしい人がいたら if anyone wants water
ii. (now here) 水を欲しがってる人がいたら if anyone wants water (meaning2)

The meaning(2) actually derives from a rhetoric that the emotion is so strong that it's visible, and that connotation is often associated with greed. So you should avoid applying ～がる to a person who is present there or one who should be respected.

この子は将来医者になりたいと思っている He wants to be a doctor in future
この子は将来医者になりたがっている*

On the other hand, this is not so bad as above when he is not there, though なりたいと思っている is better. Or, speaking of people who don't need politeness is no problem.

あの子は将来医者になりたがっていた (for some reason the present tense is still tough)
人は見たいものを見たがる
私は海外でもうどんを食べたがるだろうなぁ

viharati Wrote:私は海外でもうどんを食べたがるだろうなぁ

Despite having learned ~たがる only through passive exposure, this sentence immediately struck me as odd - seems that reading really does work.

EDICT Wrote:たがる

auxiliary verb / godan る verb

wanting to (verb) (not used in the first person, combination of ~tai and ~garu)

Yeah the only time I've seen it used in the first person is talking about the past, i.e. 子供の時、ファミコンをほしがっていた. This would tend to emphasize the "showing" aspect of -garu rather than just the wanting.

What does it mean if somebody uses the ～がる expression for their own wants? I'm fairly certain I heard a character use this in one of the Rurounin Kenshin movies. I might have misheard or heard that but misidentified the construction being used, but I'm fairly sure.

EDICT Wrote:たがる

auxiliary verb / godan る verb

wanting to (verb) (not used in the first person, combination of ~tai and ~garu)

That dictionary is wrong.

NinKenDo Wrote:What does it mean if somebody uses the ～がる expression for their own wants? I'm fairly certain I heard a character use this in one of the Rurounin Kenshin movies. I might have misheard or heard that but misidentified the construction being used, but I'm fairly sure.

ほしい or ほしいと思っている are a state that you are desirous of something while ほしいと思う or ほしがる are an instantaneous action in mind. When you mean the latter, it's what you use.

As for difference between ほしい and ほしがっている, if you dare to use the latter, it can express your wanting something could bring some influence like annoyance to others or circumstances.
e.g.
あまりにも多くを欲しがっていたら不幸になってしまうだろう If you want too much, you'd be in misery
あまりにも多くが欲しかったら不幸になってしまうだろう* (a bit unnatural)
それほどにも多くが欲しいなら努力することだ If you want that much, you have to work hard

(As I'm writing this I find it's ok to use がる form even in a situation where you need politeness as long as it's in a conditional clause.)