～たい for the second-, and third-person pronouns?
This is my original sentence, "クラスの前に彼は歌いたがっていない。"

彼は、クラスの(みんなの)前で歌いたくない。
When a native speaker corrected my sentence, she used 「たくない」. She said it sounded more natural. However, I learned that 「たい」 expresses a very personal feeling, it is usually used only for the first person. Is there any rule I can rely on to know when I can use ～たい　for the second and third person points of view?

Question

～たい for the second-, and third-person pronouns?
This is my original sentence, "クラスの前に彼は歌いたがっていない。"
 
彼は、クラスの(みんなの)前で歌いたくない。
When a native speaker corrected my sentence, she used 「たくない」. She said it sounded more natural. However, I learned that 「たい」 expresses a very personal feeling, it is usually used only for the first person. Is there any rule I can rely on to know when I can use ～たい　for the second and third person points of view?

Answer

Generally, we use ～たがっている for the third person.

In this case, I can imagine from 彼はクラスのみんなの前で歌いたがらない that "USUALLY he doesn't want to sing..." and from 彼は、クラスの(みんなの)前で歌いたくない that "NOW he doesn't want to sing...".

I hope this was helpful and you get greater answers that have more grammatical explanations.