Plurals are more than one of something, such as cows, hens and chickens. You don't have one cows. One cow - two cows. However, there are exceptions, such as one sheep, two sheep.
When we first start to learn the English language, it seems straightforward enough. A, B, C and so on. But then we come across irregular words that don't follow the rules. Our English language can be very rebellious at times and often does its own thing! Take the words good - better - best. The second two words are completely different from the first - how did it get from good to better? Plurals sometimes have a mind of their own too and it's a case of having to memorise irregular plurals until you know them by heart.
This second 11-plus spelling quiz is probably a little bit more difficult: just learn the more unusual plurals.