Classes 19-21

*Loise, Joseph, and Loise's daughter, Christobel
Joseph's sister Loise was a secondary school in Zimbabwe up until the inflation rate reached 100,000% and teachers started losing money by going to work because the transportation costs were higher than her salary. So now she is in Mozambique and had stayed with me for a week. She taught English, Shona language, and Fashion and Fabric, so I decided to take advantage of her knowledge of Shona grammar and patience to get some Shona lessons. In a matter of an hour or so she explained every single one of the twenty-one noun classes of Shona (You know how in Spanish, Portuguese, French, Italian, etc. they have feminine and masculine classes for each noun? In Shona there are 21 classes). I find the last three classes rather interesting:
Class 19: Nouns classified by the prefix svi- and referring to someone in a mocking way. For example, one would mock a boy by saying svikomana. (komana is the root of the word for boy)
Class 20: Shona speakers do not use this class because it refers to words with the prefix la- and Shona speakers do not use this prefix. (It doesn't make sense to me either, if this is their language why would they have a special class for words they don't use? Can any Shona speakers out there explain this to me?)
Class 21: Nouns classifed by the prefix zvi- and referring to someone who is fat, also in a mocking way. For example, one would refer to a fat child as zvimwana, but only if one wants to mock him because of his fatness. Thus, she said, this is rarely used.