Several years ago we had a student who was confused about ketchup and catsup. This took the staff by surprise. Being native English speakers ourselves, we had never put ourselves in the shoes of the student. We had all assumed that the difference was as clear as night and day, but were surprised to find out otherwise.
In teaching the difference between ketchup and catsup, start by making a list of flavors with the student. Ketchup is universally accepted as being “zesty” and “flavorful” whereas catsup is more “zippy” and “tangy”. After writing down the flavors, have the student associate other foods with them such as V8 juice, bisque, secret sauce, etc. The student will begin to see the clear difference between the two.
To help the student remember, engage them in a roleplay. In the roleplay, you both need to choose a restaurant for a foreign dignitary with very demanding tastes. The dignitary wishes to eat french fries, but you both need to compare the merits of ketchup as well as catsup as a topping. One person campaigns for ketchup, and the other for catsup. When finished, switch roles and try again.