Good
Incorrect: He is very good in mathematics.
Correct : He is very good at mathematics.
Note: The adjectives ‘bad’, ‘clever’, ‘quick’ and ‘slow’ are also followed by ‘at’.
For example:
I am bad at drawing.
He is very clever at making things.
She is quick at learning.
The boy is very slow at learning.
But the adjective ‘weak’ is usually followed by ‘in’.
For example:
Incorrect . He is weak at grammar.
Correct : He is weak in grammar.
