Prepositions + past continuous and past simple
He (skateboard) …. the road when he (hear) a noise.
He was skateboarding along the road when he heard a noise.
While she (run) …. the hill, she (fall) …. .
While she was running down the hill, she fell down.
They (walk) …. the river when we (see) them.
They were walking across the river when we saw them.