The car that was driving in front of me stepped on a small rock, which then came flying to my car's window and cracked it.
こんにちは。
ご質問ありがとうございます。
"The car that was driving in front of me"＝「私の前で運転していた車が」
"stepped on a small rock,"＝「小石を踏み」
"which then came flying to my car's window"＝「それが私の車の窓に飛んできて」
"and cracked it."＝「ひびを入れました。」
ご参考に。
アメリカ合衆国 
