玉米用英语复数怎么说

Corn is the uncountable noun, so it does not have a plural form in English. You can say "some corn" or "a lot of corn" to refer to multiple individual corn kernels, but you cannot say "corns" in the same way you would say "cats" or "dogs." Instead, you can use a phrase like "ears of corn" to refer to multiple stalks of corn with their kernels still attached. For example:
I picked some corn from the garden. (This means I picked some individual kernels of corn.)
I bought a dozen ears of corn at the farmer's market. (This means I bought 12 stalks of corn with their kernels still attached.)