Plural means more than one of something. Most English words become plural by adding an “s” at the end: one cat becomes two cats, one book becomes many books.

But English is full of exceptions that make plurals interesting. Some words change their spelling completely: one mouse becomes several mice, one child becomes many children. Words borrowed from other languages sometimes keep their original plural forms: one cactus becomes two cacti, one datum becomes many data. Some words stay exactly the same whether you have one or many: sheep, deer, and fish can stay the same.

The opposite of plural is singular, which means just one.

Teachers care about plurals because they affect other parts of your sentence. You write “the dog runs” but “the dogs run.” The verb changes depending on whether your subject is singular or plural. Getting this right is called subject-verb agreement, and it's one of the important patterns that makes your writing clear.