I saw a dog running across the road and knock/knocked down by a car.
Which verb in bold should I use?
Thanks.
You need more words in the sentence.
"I saw a dog running across the road and then [suddenly] it was knocked down by a car."
At a push, you could just about say "I saw a dog running across the road and being knocked down by a car" but it doesn't sound very natural.
Had you started with "A dog ran..." then it would have been "A dog ran across the road and was knocked down by a car" without needing "then it was".
"To be knocked down by" is a very specific term for being hit by a car. We wouldn't use (in BrE) anything except "knocked down by", "hit by" or "run over by".
If death was the result, we might say "He was killed by a car" but it's more usual to say "run over and killed by..."