"good in English" or "good at English"?
She's good in English.
She's good at English.
The rule
You're good AT a skill or subject, not "good in." Say "good at English," "good at maths," "good at cooking."
The adjective "good" takes the preposition "at" when you talk about a skill or subject. This is a fixed pairing — like "bad at" and "brilliant at" — so "good in" sounds wrong to native speakers.
More examples
He's really good in cooking.
He's really good at cooking.
I'm not very good in maths.
I'm not very good at maths.
Are you good in drawing?
Are you good at drawing?
How to remember it
good + at + skill (good at drawing). Keep "in" for a broader field or situation ("good in a crisis").
Frequently asked
Is "good in" ever correct?
Occasionally, in a different sense — "good in a crisis" means you perform well in that situation. But for a skill or subject, it's always "good at."
What preposition goes with "bad" and "brilliant"?
The same one — "at." Bad at, brilliant at, terrible at, hopeless at. Skill-related adjectives take "at."