There was dust on the floor.

They are quite mistaken.

They came home while I was studying.

They elected him captain.   = He was elected captain by them.

They elected me secretary. (Passive) = I was elected secretary by them.

They had dug the pond=The pond had been dug by them.

They have come yesterday. = They came yesterday.

They look forward to receiving.

They looked at the picture=The picture was looked at by them.

They vied with each other for Mary's favor.

They who are adults can use this medicine.

They, who are corrupted, caught red handed at last.

This boy is good at mathematics.

This culture is alien to our society.

This is the better of the two.