Countable nouns can be counted. In English it is important if there is ‘one’ (singular) or ‘more than one’ (plural).
The singular noun is simply the base form of that noun, for example: pen, baby, bus, potato, piano.
If there is more than one then we need to use the plural. Most plurals are formed by adding ‘s’ or ‘es’ to the base noun, for example: pens, babies (don’t forget ‘y’ changes to ‘i’), buses, potatoes, pianos.
Some nouns (but not all) which end in ‘f’ change the ‘f’ to ‘v’ and add ‘es’, for example: wolf/wolves, knife/knives, shelf/shelves.
This should be pretty easy for most of you, but you need to be careful because there are a few irregular nouns, for example:
child/children, foot/feet, mouse/mice, tooth/teeth, person/people, woman/women.
Though I think you probably know most of these too. But don’t forget that some nouns have the same singular and plural forms, for example:
sheep/sheep, fish/fish, series/series, crossroads/crossroads.
And there are some plurals which are very strange, for example:
bacterium/bacteria, cactus/cacti, criterion/criteria, fungus/fungi, medium/media, oasis/oases, stadium/stadia, vertebra/vertebrae.
And finally there are some nouns which have no singular, and are plural only, for example:
clothes, congratulations, goods, manners, police, savings, thanks.
Plus many nouns which are considered to be in a ‘pair’, which are also plural only, for example:
scissors, pants, tights, jeans, binoculars, spectacles and scales.