～がる is an extremely important suffix to overcome restrictions on person with phrases of emotional/physical state in Japanese.
Adjectival phrases of emotion in Japanese cannot take third person subjects. Meaning, you cannot say things like.
1a. Suzuki is heartbroken and sad.
To overcome this grammatical restriction, the suffix ～がる is needed.
1b. Suzuki seems sad from being heartbroken.
To conjugate, you simply attach ～がる to the stem of adjective regardless of class. Although we haven't gotten into what ～がる means, it is conveniently similar to "seem" in English. There are important differences that we'll get to later, so pay close attention!
|Sad||悲しい||To seem sad||悲しがる|
|Happy||嬉しい||To seem happy||嬉しがる|
|Embarrassing|| 恥ずかしい ||To seem embarrassed|| 恥ずかしがる|
|Scared||怖い||To seem scared||怖がる|
|New||新しい||To be fond of new things|| 新しがる|
|Rare||珍しい||To seem rare|| 珍しがる|
|Want to...||～たい||To seem to want||～たがる|
Conjugation Note: Be careful to note confuse ～たかった and ～たがった or ほしかった and ほしがった, especially when listening as it may be tricky. Note that ～がる behaves like any other verb phrase. So, if ～ている is needed, you'll need to use ～がっている.
Usage Note: It's also interesting to note that there are many adjectival phrases that ～がる is never used with for whatever reason. Some include 好きだ, 涼しい、軽い, きれいだ, etc. One restriction is that it can't be used with adjectives that show no emotional aspect of someone. There is also a subjective/objective aspect to adjectives that mustn't be ignored. When you "like" someone, that isn't a one time thing. But, いやだ could be temporary. This is one reason why you can say 嫌がる but not 好きがる.
Although this suffix is one method of getting around third person restrictions on emotion phrase (see other phrases like ～ようだ, it is not the case that ～がる is restricted to third person.
Whenever I wanted to eat ice cream, my mother would let me eat it.
Phrase Note: An easy example of debunking the claim that ～がる is used only in third person is the pattern ～がり屋, which is used to describe people's natures. So, if you're hot-nature, you're an 暑がり屋.
Using 食べたい would present an even worse grammatical error, which is using the conjunctive particle と (conditional form) with an adjective. So, what does ～がる mean? In most cases, it assesses the outward appearance or overall knowledge of something and relates the situation with the internal state of the person in question.
Ryotaro didn't let out, but he was regretting inside.
It may also be the case that it is used to help show an internal state not shown outwardly, and this can be referring to an attitude being floated by the person in question or an attitude grasped by someone else.
Ryotaro was regretting outwardly, but he was rejoicing inside.
Was I the only one who thought (that) was funny.
Note: This example shows how ～がる may be used to show a situation different than everything else.
My little brother always wants a toy when he sees it.
Note: This example shows how ～がる may be used to show someone being unreasonable.
People want to know about everyone else's situation.
Students want to know new words and expressions, but it's important to first teach the basics.
Because I'm cold-nature, life in Canada was harsh.
Because I'm hot-nature, life in Texas was extremely harsh!
You really don't have to feel sorry.
From 海辺のカフカ by 村上春樹.
My child doesn't want to eat dinner.
She wants to eat ice cream.
Everyone wants to believe in God.
Grammar Note: ～たがる is used here because this is a general statement that may not be 100% true. ～がる gives a sense that someone/ feels or thinks that way.
Aside from ～がる
Of course, there are other things you can use. For instance, ～そうだ after the stems of adjectives means "seems....". You may use かもしれない (might) or だろう・でしょう (probably). Of course, there are many other things. At the individual phase level, however, you will see that some things should just never be used for oneself and others that should only be used in reference to others.
15a. 私は怒っている。 △
She was angry at me.
Well, (he) might actually be angry at me.
Yamada-san seems so sad, doesn't she?
Attribute: No Restrictions
Words that refer to someone's wants, feelings, or likes don't need another pattern when used as an attribute/general knowledge (総合的な知識). Thankfully there are situations where the first/second and third person grammatical distinctions aren't necessary. This also goes for with situations reflecting on the past where something is already known.
The food my mother likes is sushi.
Young people want to go overseas.
My younger brother wanted to go to Korea then.
Those that want to participate, please go to the library.
In Japan, it seems that a lot of people want their fortunes read by fortune-tellers.
People that want to immediately do it are a pain.
Grammar Note: This doesn't give you a 100% free pass to not use ～がる when the situation calls for it.