Jerry is ___ Tom's left.

Jerry is on Tom's left.
Jerry在Tom的左边。

注释：
在左/右边 都用on :
on the left / right

at

ON

Jerry is ___ Tom's left.

at

at
at one's left/right
在某人的右/左边

ON

on the left of 在…的左面。 keep to the left ! 靠左边走! over the left (shoulder) 倒过来说，恰恰相反。