Verbs - Expressing Desire

I know I've had at least two other posts on this grammar topic, but Genki has some extra information on it that's important to note.

To indicate "I want to [verb]",  たい can be added to the stem of a verb to put it into the "desire" form.

学校に行きたい。
I want to go to school.

This form is conjugated like an i-adjective to indicate tense and positivity, so it's very simple.

学校に行きたくない。
I don't want to go to school.

学校に行きたかった。
I wanted to go to school.

学校に行きたくなかった。
I didn't want to go to school.

Usage of the を particle can change slightly when using the たい form. The を particle can either stay as を or become が, and the meaning of the sentence will be identical.

このジャガイモを食べたい。
I want to eat this potato.

このジャガイモが食べたい。
I want to eat this potato.

However, the たい form is generally only used when discussing your own personal desires, rather than someone else's. There are a couple ways to express someone else's desires, and one way is to quote that person.

シュウは今夜血を飲みたいと言っていた。
Shuu said he wants to drink blood tonight.

レイジは家に遊びに来たいと言っていた。
Reiji said he wants to come over to play.

Another way is to use a new verb - たがっている. The original form is たがる, and it essentially means "I think this person wants to do X because of the way he's behaving." This verb is added to the stem of the original verb, just like with たい.

It's important to note that when using たがっている, the を particle must be used. It would not be grammatically correct to use が like we can with たい form.

ライトは起きたがっているね。
Laito wants to go to bed.

スバルは美味しい晩ごはんを焼きたがっている。
Subaru wants to cook a delicious dinner.

リーさんは合格したがっているよね。
Lee-san wants to pass the test.

アヤトはあの本を読みたがっているね。
Ayato wants to read that book.