～がる
show signs of feeling
～がる attaches to adjectives of emotion or sensation and turns them into verbs meaning “to show signs of being ...”. Drop the final い of an い-adjective, or take the stem of a な-adjective, and add がる: 怖い becomes 怖がる, “to act scared”.
Japanese treats inner feelings as something only the person who has them can state directly. You can say 私は怖い about yourself, but a third person's feeling is normally reported through its visible signs with ～がる — roughly “he acts scared” or “she seems to want it”.
1. Reporting another person's emotion
Note the particle shift: the adjective takes が (犬が怖い), but the がる verb marks the same thing with を. The plain form describes a general tendency, while ～がっている describes the feeling being shown right now.
弟は犬を怖がる。
“My little brother is afraid of dogs.”
- 弟（おとうと）= younger brother
- は = topic marker
- 犬（いぬ）= dog
- を = marks the cause of the feeling (replaces が with がる)
- 怖がる（こわがる）= to show fear of, to be (visibly) afraid of
2. ～たがる: what someone else wants to do
がる also combines with the desire form ～たい to give ～たがる. Where ～たい states the speaker's own desire directly, ～たがる reports the desire another person is showing, and it very often appears as ～たがっている.
妹は新しいゲームを買いたがっている。
“My little sister wants to buy a new game.”
- 妹（いもうと）= younger sister
- は = topic marker
- 新しい（あたらしい）= new
- ゲーム = game
- を = object marker
- 買いたがっている（かいたがっている）= is showing she wants to buy
How to form it
い-adjective stem (drop い) + がる / な-adjective stem + がる / ～たい → ～たがる
- 怖い（こわい）= scared → 怖がる（こわがる）= to show fear
- 欲しい（ほしい）= wanted → 欲しがる（ほしがる）= to show that one wants
- 嫌（いや）= unpleasant, unwilling → 嫌がる（いやがる）= to show dislike
Key idea
～がる shifts the viewpoint from inside to outside: it describes the behavior a feeling produces, not the feeling itself. That is why it is the natural way to talk about other people's emotions and desires in Japanese.
Other N3 grammar
Updated .