be good at的反义词可以是：be bad/poor at..； be weak in...；表示不擅长或在某些方面做的不太好。
be bad/poor at sth. 不擅长某事
be bad/poor at doing sth.不擅长做某事
be weak in sth. 不擅长某事
be weak in doing sth. 不擅长做某事
Mr Green is bad/poor at sports.
=Mr Green is weak in sports.
Tom was bad/poor at Chinese.
= Tom was weak in Chinese.
Mathew is weak in public speaking.
=Mathew is bad/ poor at public speaking.
Andrew is bad/poor at swimming.
=Andrew is weak in swimming.
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be good at的用法