得意でない

not good at

→ 得意だ
phrase Uncommon #15,726
not good at A somewhat formal or written way to say 得意じゃない. Usually used with が or のは to indicate the activity or subject someone is not good at.
JA: 私は人前で話すのが得意でない。
EN: I'm not good at speaking in front of people.

彼は自分の考えを言葉にするのが得意でない

He isn't good at putting his thoughts into words.

私はスポーツ得意でない

I'm not good at sports.

私はトランプ得意でない

I'm not good at card games.

私は物事を分類するのが得意でない

I'm not good at categorizing things.

スポーツ得意な人が得意でない人を助けていました

Someone who was good at sports was helping someone who wasn't.