Both are correct according to my understanding : It was a car that crashed into that milestone. - It means, the car was unidentified , and you know nothing about it , it is just an information . -- Here, it was a particular car about which you are talking.
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This one:
so, if I have a main sentence like this
A car crashed into that milestone.
How do you change it?
It was a car that crashed into that milestone. or It was the car that crashed into that milestone