A – An – Some
A and an can only be used with nouns in the singular if we don't mean a special thing or person. Some can only be used in the plural if we don't know the exact amount of something or if we use nous we can not count, e.g. liquids.
Singular: a or an
Do you have a ball? - not a special ballShe doesn't have a sister. - not a special sister
Plural: some with countable nouns
Buy some tomatoes, please. - not a special amountThey have some children, haven't they? - we don't know how many
Use some for things you can't count
Would you like some orange juice? - May I have some more honey?We can't count juice or honey - only glasses, litre, tins, kilos, ...