Cards have words at the top, and arrows pointing from one card to the next.
Some cards can be rotated so either of two words can be on top, like "have" and "has".
When the words on top form a grammatically correct sentence, all the arrows will line up.
Arrows will not match between words that don't go together, like "they sees" or "an bicycle. Many sentences can be corrected by rotating one card.
I expect to add more pages soon, but I'll appreciate comments at any stage.