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Can I say,

(i) At / IN the beginning of the race, the hare ran very fast.

9ii) The hare ran very fast At / IN the beginning (of the race)
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I would use 'at', reserving 'in' for Genesis.
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Vincent TeoCan I say,

(i) At / IN the beginning of the race, the hare ran very fast.

9ii) The hare ran very fast At / IN the beginning (of the race)

A reasonably good rule of thumb: if of occurs after beginning or end, use at:
At the beginning of the race, at the end of the race
If of is not used, use in:
Everything seemed easy in the beginning. He became happy in the end.
CB
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Comments  
I'm a big fan of your rules, CB including this one!!! You make my life easier.
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New2grammarI'm a big fan of your rules, CB including this one!!! You make my life easier.

My pleasure! Emotion: smile CB