Question

1. I beat him at pingpong.
2. We defeated them at basketball.
3. He beat the big man at wrestling.
4. I defeated him at bowling.
5. We defeated the opponent team at basketball.
6. They beat us at soccer.
7. I was beaten at chess by him.
8. I was defeated at tennis by her.
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Are they all grammatical? Do we have to use 'at' alone before game names?

Answer 1

Sometimes "in" can be used instead of "at" but using "at" makes the competition clearer.

In 5, I'd say "the opposing team" -- but all the rest are fine.

Answer 2

Thank you for your help.
What about the following?

1-1. I beat him at a pingpong match.
1-2. I beat him in a ping-pong match.

2-1. We defeated them at a basketball match.

2-2. We defeated them in a basketball match.

(Which ones are correct? Are they all grammatical?)

Answer 3

1-1 should be "I beat him at ping-pong."
1-2 is fine.

2-1 should be "We defeated them at basketball."
2-2 is fine.

Do you see the difference?