One day,he saw a cat behind a wonan when he was going to work by a car是对是错?错了如何改?

错了，应改为“One day,he saw a cat behind a wonan when he goes to work on a car”
(1)when 和if引导的重句后面是一般现在时，所以要把was going改为goes.
(2)没有by a car这个句型，只有by car和 on a car，by后不能有任何介词，而by car指坐的交通工具，on a car指过程。所以应该向我这么改

in a car
或者by car

by a car->by car

by a car改by car
还有，he saw a cat is behind a woman

1)when改成while比较好
2）by car就行了，不用by a car, by后面不能加a

by a car改成by car